In the field of polarization imaging and laser power measurement, an obvious error can be produced due to the effect of light polarization state on a photodetector in any polarization-dependent quantitative measurement. This phenomenon is mainly caused by the fact that many kinds of photodetectors are sensitive to light polarization. In this paper, the polarization-dependent responsivity (PDR) of a photodetector is analyzed, the stokes model of polarization-dependent loss (PDL) is deduced and the effect of a SiNx passivation film on the PDR of an InGaAs photodetector is analyzed. The analysis result shows that when the SiNx passivation film is not used, the PDL increases with the increasing of an incident angle and decreases with the increasing of wavelength; and when the SiNx passivation film is used, the PDL firstly decreases and then increases with the increasing of wavelength. Because the central anti-reflection wavelength of the designed SiNx passivation film is at 1550nm, the PDL at the wavelength of 1310nm is greater than that at the wavelength of 1550nm.