Abstract: Considering the nondeterministic order of the separation results as well as lack of separation effectiveness measurement approach in blind separation of underwater target echo and reverberation for active sonar, the signal instantaneous frequency feature as the indicator is proposed to be a performance evaluation method. The derivation of the time-frequency distribution character of target echo and reverberation shows that, the center deviation and the randomness of the instantaneous frequency sequence of target echo is lower than those of reverberation. Thus, the signal instantaneous frequency variance and instantaneous frequency entropy as two signal features are extracted to be the evaluation of target echo identification in blind separation results. The sea experimental results have shown that the target echo has the minimum instantaneous frequency feature value among all the separated signals after blind separation, while with smaller feature value, the better separation results of target echo and reverberation will be obtained.