Abstract: This paper presents a method of phase test based on a spectrum analyser. Test principles are analysed, application in an automatic test system of power amplifying modules of secondary radar is described,and application cruces are proposed, which are the selection of phase shifters, the settings of the spectrum analyser, the algorithm realization, and the usage of relative radio frequency adaptors and a reference line. Comparison test versus VNA is done, and the result difference is analysed from three aspects, which are the accuracy of phase shifters, the state deviation where the minimum power is, and the difference in testing conditions. Since the test results by the two methods are close, considering the advantage in costs,the method proposed by this paper can be another choice of phase tests.