Abstract: In the field of microwave test, researchers mainly focused on the theory of power supply ripple, but didin′t specifically target to the impact of power supply ripple to the phase noise of frequency source, and with even less case studies. As a result, current advances can not provide any clues to help designing electromagnetic compatibility for microwave circuit. It′s expressed in this paper about the study targeted the effect of power supply ripple on spectrum purity of frequency synthesizer in modern radars, on the basic of the conceptual analysis about the effect of power supply ripple on spectrum purity. The principle about the effect of power supply ripple on spectrum purity were analyzed. Then the effect on different types of frequency synthesizers through simulating data simulation were explored. And the main factor and correlation of the effect of power supply ripple on spectrum purity of frequency synthesizers after comparing the results of actual testing and simulating were got, the date was offered to help the circuit designers.