The crosstalk in plastic scintillating fiber array detector with highenergy gamma incident was studied through MonteCarlo simulation. The relationship between crosstalk and lead layer was also analyzed. Modulate transfer function(MTF),which shows quantity of spatial resolution of imaging detector, was analyzed and compared in order to get the MTF results caused from the difference thickness of lead layer. The results show that there exists severe crosstalk in the detector irradiated by highenergy particles, and the lead layer can reduce the effect. On the other hand, the pixel of detector may become big as the lead layer is introduced, which can reduce the spatial resolution. Through the simulation computation, it is proved that the right thickness of lead layer will reduce crosstalk and increase spatial resolution of imaging detector.