The piezoelectric micromachined ultrasonic transducer (PMUT) has important applications in medical imaging, gesture recognition, endoscopy imaging, fingerprint recognition and other fields, and its performance such as sensitivity are the main factors affecting the imaging quality. This paper analyzes the equivalent circuit model of PMUT based on Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT)circular piezoelectric composite vibration film, and then investigates the influence of PMN-PT film thickness on the transmitting voltage response and receiving sensitivity of PMUT by finite element method. The simulation results show that when the thickness of the piezoelectric layer is 4.5 μm (90% of the thickness of the substrate), the value of the transmitter voltage response level of the transducer is the maximum, reaching 191.6 dB. The receiving sensitivity basically increases linearly with the increase of thickness. When the thickness of the piezoelectric layer is 5.1 μm (102% of the thickness of the substrate), the number of loop gain (loss) is the maximum, reaching -64.50 dB.