A new analytical model for photoconductive semiconductor switches (PCSS) is proposed considering the surface and bulk recombinations, the carrier-carrier scattering and the negative differential effect of the carrier drift velocity in their transport process, Dembet effect, optical reflection and attenuation with depth during the optical action. Some important parameters characterizing PCSS, such as on-resistance and switching efficiency etc, have been simulated. Also the waveforms of the switch current and output voltage have been obtained. The calculations show that the relation between photo-conductance and incident ligh intensity is not strictly linear but nonlinear in what is called linear operational mode of the switch. The good agreement has been obtained by comparing the calculation with the experimental data.