We establish a three-dimensional (3D) mathematical model based on the finite element method and calculate the effect of the Ag nanoparticle arrays that are periodically distributed on the surface of the silicon thin-film photodetector on its light absorption performance. The results denote that, for the spherical Ag particle arrays, the key parameter affecting the light absorption efficiency of the silicon film is the ratio of the period to particle diameter. When the top particles of the silicon substrate are densely packed (i.e., when the ratio P/d is small), the absorption efficiency of the plasmon photodetector is improved to different extents at different incident angles when compared with that of the bare silicon photodetector. The absorption efficiency increases from 5% to 65% at the wavelength of 700 nm with the incident angle ranging from 0° to 65°. Furthermore, the photoelectric conversion efficiency increases from 29% to 34% over the entire wavelength range.