Aiming at the problems of small reciprocating motion displacement and low displacement amplification ratio in the existing resonant diaphragm pumps driven by piezoelectric vibrators, a piezoelectric stack actuator with large displacement is designed in this paper. The piezoelectric stack is used as an electro-mechanical converter to improve the response speed, and a rectangular spring structure is adopted to ensure the stability of the system and improve the first order natural frequency of the system. The mathematical model of piezoelectric stack actuator is established, the displacement amplification ratio and natural frequency of each order are analyzed, and the related performance of the prototype is tested. The experimental results show that the output displacement of the piezoelectric stack actuator is 154 μm and the displacement magnification is 18.11 times under the sinusoidal signal with the input excitation amplitude of 60 V and frequency of 510 Hz. The piezoelectric stack actuator has high response speed and large output displacement, and can be used to improve the output performance of resonant diaphragm pump.