The rapid turn-on process of nonlinear photoconductive semiconductor switch is always associated with the formation of filamentary current. Some turn on characteristics of nonlinear photoconductive semiconductor switches are analyzed based on the assumption that the mechanism of the turn on in photoconductive semiconductor switches is the streamer discharge. The results, with carrier lifetime, optical absorption length, impact ionization coefficient, initial carrier concentration considered, fit with the experimental results well. This discussion corroborates the assumption and is helpful to the full understanding of the physical processes occurred in nonlinear PCSS. Some methods of improving and controlling the operational characteristics of PCSS are also given.