Poly-L-lysine is an important polycation which has great prospect in the biomedical application. However, there are few reports on the blood compatibility of poly-L-lysine at present, especially the interaction between poly-L-lysine and some important proteins from the blood, which is studied with spectroscopic methods. Therefore, it is of certain innovation to further evaluate the blood compatibility of poly-L-lysine, which is studied by the interaction between poly-L-lysine and fibrinogen with many spectroscopic methods. In this study, the fluorescence, ultraviolet visible and circular dichroism spectroscopy were used to research the effects of poly-L-lysine on the structure of fibrinogen. Briefly, zeta potential experiment showed that positive potential of poly-L-lysine increased with increasing concentration. Complexation experiment showed that 0.01 mg·mL-1 poly-L-lysine had little effect on the function of fibrinogen, and the interaction strengthened with concentration increasing. Fluorescence spectra showed that the fluorescence of fibrinogen was quenched in a concentration dependent way when the emission wavelength was 341 nm (λem=341 nm). Ultraviolet visible spectra showed that the absorption intensity of fibrinogen at 200～240 and 278 nm, which was less affected by 0.025 mg·mL-1 poly-L-lysine, reduced with the concentration of poly-L-lysine increasing. Circular dichroism spectra showed when the concentration of poly-L-lysine increased, the effect of poly-L-lysine on the secondary structure of fibrinogen strengthened. Overall, as the concentration of poly-L-lysine increased, the α-helix content of fibrinogen decreased and the β-fold, β-turn and random coil content of fibrinogen increased. These results showed that the electrostatic interaction was occurred between poly-L-lysine and fibrinogen, which influenced the structure of fibrinogen by concentration dependence. While the effect of poly-L-lysine (0.01 and 0.025 mg·mL-1) on fibrinogen was little, poly-L-lysine of high concentration would damage the physiological function of fibrinogen. Therefore, it is necessary to consider the concentration factor in the development and application of poly-L-lysine. In this study, a simple, convenient and systematic method is utilized to study the interaction between materials and fibrinogen, which is helpful to evaluate the blood compatibility of materials sufficiently. Furthermore, the results of this study will provide important guiding information for the biomedical applications of poly-L-lysine.