Abstract: Sometimes we can not obtain the interference signal information, but can easily obtain the input signal and output signal information when FM signal has got single-tone interference. So we propose to modify the mean square error and make it be associated with jamming to signal ratio(JSR), then we evaluate indirectly the FM signal single-tone interference. First, we establish the simulation model of FM signal and analyze distortion of FM demodulated signal under different SNR and different JSR. And we analyze the relation between JSR and mean square error which is used to express analog signal distortion. Then we modify the formula of the mean square error and obtain the new evaluation index after the simulation. Finally, we analyze the relation among the new evaluation index, the JSR and the duty cycle and get the conclusion.