A PSD is modeled as an approximate Linear Time Invariant (LTI) system based on the rc transmission line model and the impulse response obtained from Lucovsky equation. Based on the model, the PSD general response is derived. In our processing, a time-varying light signal is divided into many sections in time domain. Each section is regarded as a square-wave pulse. And the response is approximated to be a linear superposition of those pulse responses. Results show that the response is the summation of product of two factor sequences: one factor is the inherent decay factor and the other is the outer incentive response factor. In addition, analysis on Alternating Current (AC) response by virtue of the general response shows that there is hysteretic fluctuation with light intensity change and the current response is insensitive to high-frequency AC optical signals.