Abstract: With the rapid development of modern electronic countermeasure technology, more and more attention is being focused on the active decoy technology for anti-radiation missile(ARM). It is an effective method to use active decoy against ARM. In this paper, the mathematical model formula of non-coherent active decoy confront ARM is derived. According to the ARM radar seeker before and after the arrival of the space separation point, an active decoy model is proposed. And the decoy effect on the non-coherent two point source, three point source, four point source decoying in considering space separation point and not under the two conditions are simulated and analyzed. The results show that when the space separation point is considered, the effect is not only better, but also more close to the reality.