In order to improve the performance of signal processing system in traditional radar, the IF digital quadrature demodulation and the coherent signal getting method is introduced The improving method of coherent detection for coherent-on-receive radar is studied, analog coherent detector is replaced by IF sampling and digital quadrature demodulation, digital mixer and filter is used to get the original phase of transmitting signal. Simulation is made with real signal of coherent-on-receive radar, and the result indicates that the signal processing effect is greatly improved and shows that this new method is valuable for the digital update of some early stage radar and the design of modern radar signal processing system.