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In order to find the interpretation of the incomplete closure of piezoelectric pump check valve in high frequency, the characteristic of the pump in that working conditions was analyzed, together with the judgment basis and theoretical demonstration of the existence for the phenomenon. The incomplete closure of piezoelectric pump check valve phenomenon was found, and frequency response of the check valve in water was obtained from its dynamic model. The mean force and displacement of valve was calculated in hydrodynamics. Relative reaction of the frequency response and mean displacement were analyzed. Sufficient condition for the check valve incomplete closure was obtained which was amplitude amplification less than 0.25. The results show that check valve can close completely operating at 80Hz. When the amplitude amplification less than 0.25,the check valve incomplete closure phenomenon was observed at 140Hz. With further increasing of the frequency, amplitude amplification factor decreases, the phenomenon is more obvious under the condition of low amplitude amplification factor, the minimum clearance between the check valve and valve seat more than 30μm at 260Hz.