Based on the analysis of principle and requirements of four-point source decoy system against anti-radiation missile (ARM), the derivation and calculation of amplitude and phase of the compound electric field at the passive radar seeker (PRS) of ARM under four-point source decoy system were given. Then the simulation was done to point out the relation of the amplitude and phase of the compound electric field at the PRS and the distance between ARM and target radar on the changing condition of the attacking process of ARM. Finally, the simulation conclusion was analyzed to give the theoretic support to the decoys' location and the analysis of the decoys' efficiency of the four-point source decoy system.