A simulation of dynamic nonlinear characteristics of high voltage optically controlled photoconductive semiconductor switches (PCSS′s) is performed with 2 D simulator MEDICI, and the emphasis is put on the mechanism of lock on and time delay under high electric field condition. It is found that the deep energy levels play an important role in lock on and time delay. They can dynamically change the distribution of electric field, carriers and current densities in PCSS′s, cause output current to delay and also strengthen the local electric field enough to cause avalanche. The simulated results agree with experimental results. It is concluded that the observed nonlinear characteristics are closely related to the introduced deep energy levels in PCSS′s.