Abstract: It is valid to improve the system performance of passive radar by utilizing multiple illuminators for detection and position, drawing on the idea of orthogonal signal Multiple-input Multiple-output(MIMO) radar. Aimed at the passive radar based on FM broadcasting signal, this paper researches the signal model and the principle of coherent synthesis based on multiple FM radio channels, analyzes the phase relation of each signal, and gives the phase items that influence coherent synthesis. The study shows it is feasible to realize coherent synthesis of signals from multiple illuminators and improve the signal-noise-ratio (SNR) of passive radar systems, by removing the differentia phase item of each signal.