Diamond has a wide range of applications in the field of precision and ultra-precision machining of hard materials, high frequency and high voltage electronic devices, chip thermal management, and precision optics due to its extremely high hardness, excellent thermal conductivity, extremely high electron mobility, and wide-band optical transparency. Furthermore, it has been demonstrated that the preparation of microstructures on diamonds can significantly enhance the performance of diamond-based devices. However, it is difficult to efficiently prepare microstructures on diamond surfaces using conventional mechanical or chemical methods. Laser processing has become an advanced method to fabricate diamond microstructures due to its low cost, simple process, non-contact machining, high flexibility and high efficiency. Currently, most studies are focused on the optimization of processing parameters and the preparation of diamond microstructures. However, little attention has been paid to the effect of laser parameters on the micro/nano-structures in the laser-ablated area, and the material removal mechanism of the femtosecond laser processing diamond is not clear. Here, in this work, the effects of laser processing parameters such as laser fluence, scanning speed, and number of scans on the microscopic morphology of diamond microgrooves were investigated. The ablation threshold, ablation rate, and material removal rate of diamond processed by femtosecond laser were further obtained. Then the phase transformation of the diamond under femtosecond laser irradiation was analyzed through Raman detection. The effect of laser fluence on the Raman spectra which was detected at the central region of the diamond microgroove was further investigated. The residual stresses in the center of the diamond microgrooves under different laser fluences were calculated based on the wave number shift of the diamond peak. Finally, the temperature field of femtosecond laser-irradiated diamond was simulated using ANSYS finite element software, and the removal mechanism of diamond material was analyzed. It was found that femtosecond laser ablation of diamond microgrooves resulted in a clean surface and debris-free edges, but micro-cracks along the $〈\mathrm{110}〉$ crystal orientation appeared in the laser-machined area. Periodic nano-ripples were formed within the microgrooves. The topographies of nano-ripples were closely dependent on the laser processing parameters. As the laser fluence increases, the order and uniformity of the nano-ripples in the microgroove first become better and then gradually become worse. With the increasing laser scanning speed, the periodicity of nano-ripples in the center of the microgroove becomes larger, and the nano-ripples at the edge of the microgroove become discontinuous and uneven. As the number of laser scans increases, nano-ripples with a periodicity close to half-wavelength appear in the center of the microgroove. However, with the continuous increase in the number of scans, a large amount of heat accumulation is imposed, leading to irregularities in the nano-ripples at the center of the microgrooves and poor uniformity of the nanoripples at the edges of the microgrooves. The ablation threshold of CVD single-crystal diamond was experimentally calculated to be 3.20 J/cm². When the laser fluence was increased to 24.34 J/cm², the ablation rate of diamond was 44.8 nm/pulse, the material removal rate was 4.34×10^-10 g/pulse, and the residual tensile stress was increased to 1 389 MPa. The Raman detection revealed a very thin graphite layer at the bottom of the microgroove, and the thickness of the graphite layer was theoretically calculated to be 11.1 nm. Finally, Simulation results illustrate that the temperature at the center of the femtosecond laser beam has far exceeded the temperature of diamond sublimation. In addition, the femtosecond laser energy irradiated on the diamond is mainly distributed on the diamond surface, while it conducted to the inside of the diamond is very little. The thickness of the graphite layer obtained from the simulation is very close to the theoretical calculation.