The influence of CH3CSNH2 passivation on diffusion at interface between ferromagnetic metals and GaAs has been studied. The experimental results show that sulfur passivation can change the chemical environment of As element , prevent As from diffusing into ferromagnetic metal overlayer, weaken the reaction of As with ferromagnetic metals and enhance the magnetism of the interface. Furthermore, we discussed the reason that S passivation can prevent As atoms diffusion.