Monte-Carlo method is adopted in GaAs PCSS's simulation, In the case of high optical fluence, space-charge field can intensity influence the movement of the carrier. Thus, space-charge field can intensity influence not only the shape of photo-electric current of PCSS's, but also the terahertz out put of photo-conducting antenna. In this paper, the forming and movement of space-charge field are simulated by means of Monte-Carlo method. And the result of simulate indicates that optically activated charge multi-domain exists in photoconductor. The forming of multi-domain is also explained in this paper.