In this paper, the phenomena of plasma oscillations in silicon-based p-n junction photoelectric detector are researched. Starting from the classic Drift-Diffusion Model, the basic equations of photodetector with reverse bias under the radiation of femtosecond optical pulse were deduced. In our physical model, the carrier mobility in low electric field was introduced, and basic parameters including diffusion coefficients and damping coefficients were modified according to the nonlinear relation between carrier drift velocity and high electric field. A numerical algorithm base d on the finite difference method is proposed to solve the model. By solving the equations numerically, we obtained the transient dynamic behaviors of this kind of photoelectric detector, the current responses of the plasma oscillations phenomena, and the frequency of plasma oscillations, etc. By comparing the numerical solutions of plasma oscillations with approximate analytical solutions, we explored the reason for the difference between them.