Abstract: In order to solve the problem that the spaceborne power amplifier noise power ratio index lacks the assessment method, this paper is based on the principle of multicarrier nonlinear operation, by using the method based on digital I/Q modulation to edit the baseband waveform file with Matlab to form a wideband multicarrier signal with notch, and calibrating the amplitude flatness of wideband multicarrier signal according to the digital preprocessing method of frequency spectrograph sampling, and adjusting the I/Q power offset of vector signal generator to change the depth of notch, the wideband multicarrier excitation signal satisfying the test conditions is finally generated. By inputting the excitation signal to the power amplifier and analyzing the intermodulation distortion components generated in the notch of the output of the power amplifier under test, a test scheme is designed by means of vector signal generator and spectrum analyzer. According to the test system built by the scheme, this test method to a K-band traveling-wave tube amplifier (the main carrier frequency is 19800MHz, the bandwidth is 80MHz) is tested and verified. The results show that the noise power ratio test method proposed in this paper is effective and feasible, and provide a specific test method for evaluating NPR index of spaceborne microwave power amplifier.