Several significant characteristic quantities as well as physical mechanism in high gain intrinsic GaAs photoconductive semiconductor switches (PCSS) are analyzed. The model of the streamer on the basis of domain electron avalanche (DEA) is developed. It is presented that the DEA requires three necessary conditions:the lower limit of initial carrier density nLD≥3×1015 cm-3,the local carrier density at least either in the depletion layer or in the accumulation layer of the DEA must exceed the threshold ncs,the characteristic length ΔZ of the activated region along the field must be larger slightly than the width b of the growing domain. The upper limit of the local carrier density created by photo-ionization from the streamer is approximately 1017 cm-3·ps-1,which implies that the carrier density provided for the successive growing domain is approximately 3×1015≤n(t＝0)＜1017 cm-3. Therefore the propagation velocity of streamer is approximately 2.97×108≤vpro≤6.21×109 cm/s. The results of this theoretical analysis are consistent with the reported experimental observations.