Abstract: The optimum detector for a determined signal in the stationary white noise background in ideal channels is a matched filter. But its property degenerates seriously in a practical underwater acoustic system for the complexity of underwater acoustic channels. Based on the estimation of instantaneous frequency sequence and its statistical characteristics, a new type of robust detector -- Variance -of-Instantaneous-Frequency Detector (VIFD for short ) is presented for the detection of CW pulse according to the features of underwater acoustic channels and operating environment of active sonars. It is proved through theoretical analysis, simulation and experiments that VIFD is promising for its robustness in nonstationary interference background and channels and that it is also robust for the fluctuations and decay of signal or mismatch caused by Doppler-frequency shift.