Abstract: Affecting the performance of needle opening-close in fuel injector,the displacement of piezoelectric multilayer stack actuator (PMSA) was analyzed by test measurement to find the position of displacement wavelet,and some factors causing the displacement wavelet of PMSA were analyzed.Firstly,a simulation model was established for the PMSA based on ABAQUS software to analyze the displacement change of PMSA in different time.Results show that the piezoelectric component inertia is the main reason causing displacement wavelet of PMSA in the expansion and contractive stage.Then a hydraulic vibration absorption method and a voltage optimization method were presented to decrease the displacement wavelet of PMSA.Experimental results show that hydraulic coupling displacement amplifier can reduce the displacement wavelet to a certain extent,and trapezium current control can effetely reduce displacement wavelet of PMSA.