Fiber lasers have a wide range of applications in material processing, biomedical, industrial production, military security and other fields with high electrical-optical and optical-optical conversion efficiency, variable wavelength range, high quality of output beam, compact structure, and low cost, which becomes the research hotspot in recent years. Fiber lasers with the ultrafast laser pulse are mainly realized by active mode-locking and passive mode-locking methods. Compared with the active mode-locking technology, the passive mode-locking has attracted more attention on account of its advantages such as narrower pulse output, compact structure and convenient operation, in which saturable absorber is the core component. The modulation depth and saturation fluence of saturable absorbers determines the output characteristics of passively mode-locked lasers. Therefore, it is necessary to explore excellent saturable absorbent materials. Ge₂Sb_1.5Bi_0.5Te₅ materials have a wide spectral response, high thermal, chemical and mechanical stability, and can achieve rapid transition between amorphous and crystalline states at low temperature. Ge₂Sb_1.5Bi_0.5Te₅ materials have been frequently used in laser direct writing technology as an outstanding photoresist, but its research in the field of photonics is still in its infancy. To our best knowledge, Ge₂Sb_1.5Bi_0.5Te₅ materials have not been used as the saturable absorber for the generation of the ultrafast laser pulse. Therefore, the exploration of optical properties and applications of Ge₂Sb_1.5Bi_0.5Te₅ could promote the comprehensive understanding of Ge₂Sb_1.5Bi_0.5Te₅ materials and effectively drive the development of optical devices based on Ge₂Sb_1.5Bi_0.5Te₅ materials. Here, Ge₂Sb_1.5Bi_0.5Te₅ films with the thickness of 40 nm were prepared by the magnetron sputtering method on a gold mirror under the atomspheric pressure of 0.1 Pa, the power of 50 W and the Ar flow of 25. Then the Ge₂Sb_1.5Bi_0.5Te₅ films were annealed at 150 ℃ for 20 min in vacuum tube annealing furnace. X-ray diffraction analysis of Ge₂Sb_1.5Bi_0.5Te₅ films shows that the as-grown Ge₂Sb_1.5Bi_0.5Te₅ films are amorphous and after annealing the crystalline state has been observed in Ge₂Sb_1.5Bi_0.5Te₅ films. The optical absorption of crystalline Ge₂Sb_1.5Bi_0.5Te₅ films in the range of 1 300~1 600 nm is significantly higher than that of amorphous Ge₂Sb_1.5Bi_0.5Te₅ films, and the absorbance of amorphous and crystalline Ge₂Sb_1.5Bi_0.5Te₅ films at 1 550 nm are 5.61% and 21.16% measured by spectrophotometer, respectively. A typical balanced twin-detector test system is used to explore the nonlinear absorption characteristics of amorphous and crystalline Ge₂Sb_1.5Bi_0.5Te₅ saturable absorber. The modulation depth of the amorphous Ge₂Sb_1.5Bi_0.5Te₅ saturable absorber is 2.7% and that of crystalline Ge₂Sb_1.5Bi_0.5Te₅ after annealing increases to 3.8%, which is 1.4 times higher than the amorphous Ge₂Sb_1.5Bi_0.5Te₅ saturable absorber. The reason for the improved nonlinear absorption capacity of crystalline Ge₂Sb_1.5Bi_0.5Te₅ films can be mainly attributed to the formation of the long-range ordered structure with less unsaturated bonds and less local defects in the crystalline Ge₂Sb_1.5Bi_0.5Te₅ saturable absorber. Moreover, the crystalline Ge₂Sb_1.5Bi_0.5Te₅ saturable absorber also exhibits the smaller band gap which decreases the binding energy of excitons. The lower binding energy of excitons slows the recombination efficiency of electron hole pairs, and the modulation depth of saturable absorber is inversely proportional to the recombination efficiency of electron hole pairs. Therefore, crystalline Ge₂Sb_1.5Bi_0.5Te₅ with a smaller band gap has a greater modulation depth. Based on the crystalline Ge₂Sb_1.5Bi_0.5Te₅ saturable absorber, an Er-doped fiber laser system has been constructed to obtain the ultrafast laser pulse in which a semiconductor laser with a central wavelength of 980 nm is used as the pump source and is connected to a wavelength division multiplexer at 980/1 550 nm. A 1.3 m Erbium-doped fiber is employed as the gain medium. The polarization independent isolator ensures the unidirectional operation of the signal light in the cavity, and the polarization controller is used to adjust the polarization state in the cavity to affect the mode-locked state. The laser beam is applied to saturable absorber through the ring device and re-coupled back to the ring cavity to generate a mode-locked laser pulse. Then the laser pulse passes through a fiber coupler with a beam splitter ratio of 90∶10. 10%of the output laser is used to observe the performance of the laser in real time, and the remaining 90% of the laser is used to continue oscillating in the cavity. The obtained output pulse width is 1.52 ps, the repetition frequency is 3.44 MHz, and the signal-to-noise ratio is 47 dB. In addition, to investigate the effect of film thickness on the absorption properties, Ge₂Sb_1.5Bi_0.5Te₅ films of 60 and 80 nm were prepared by magnetron sputtering. The experimental results of the UV-Vis-NIR absorption spectra show that the optical absorption increases with the increase of Ge₂Sb_1.5Bi_0.5Te₅ films thickness, which indicates the controllability of optical properties of Ge₂Sb_1.5Bi_0.5Te₅ films and reveals the great potential of Ge₂Sb_1.5Bi_0.5Te₅ materials in ultrafast lasers. Our finding can provide reference for the application of phase transition materials in photonics and help for the wide application of fiber lasers.