Abstract: A laser wire system is being developed in the electron beam transport line of BEPCⅡ. In this paper, the simulation based on GEANT4 is presented. Firstly, the electromagnetic shower distribution in detector material was simulated to choose an initial detector dimension for Cerenkov process simulation. Then detector responses for different dimensions, Compton signal positions and beam energy were obtained, and the most important result is that the detector dimension is determined at which an optimal signal output can be achieved. The results for different Compton signal positions show that the effect of the beam pipe is significant, which is expected to be important for the beam profile measurement. Furthermore, the simulation results of the detector response and electromagnetic shower energy leakage at different electron beam energy are presented.