Phase modulation of multiple coherence have important application foreground in electronic warfare. This paper is aimed at discussing the influence of the plane matrix multiple coherence on the oscillation characteristic of phase modulation. Because of its special background of application, we knew that the study of this problem hasn't been reported. The similar study on the problem is coherent combining of fiber laser. The significance of phase modulation is to make the phase of every coherence unit time variation rule so that it can realize special purpose in actual application. When we set up the model of the numerical simulation, we take harmonic phase modulation for the sake of simplicity but to illustrate this problem.