Abstract: In this paper, the frequency domain characteristics of the traveling wave tube(TWT)output signal based on pulse modulation are analyzed, and the application of the vector network analyzer in pulse-modulated signal testing is discussed. Two methods of wideband detection and narrowband detection can be realized by setting the receiver bandwidth of the vector network analyzer; the pulse desensitization effects of the two detection methods are compared with the analysis of typical pulse modulation signals, the gain characteristics of pulsed TWT under different test methods are analyzed, and the wideband detection method is established as the test method of gain fluctuation of the TWT. The actual test results show that the wideband detection method of the vector network analyzer is more suitable for measuring the gain fluctuation of pulsed TWT.