Abstract: As one of the important performance parameters of the signal source, it is of great significance to measure the signal source voltage standing wave ratio（VSWR）accurately. Based on studying the structure of the signal source and the two working mechanisms of “RF OFF”, the influence of "RF ON" and "RF OFF" on the measurement results of VSWR for the signal source was analyzed. With a vector network analyzer as the measurement equipment, this paper studies the measurement methods of VSWR for signal sources in different modes. The experimental results showed that the passive device measurement method under the condition of “RF OFF” can be used for the signal source with fast closing mode, while the frequency offset measurement method under the condition of “RF ON” can be used for the complete shutdown mode.