A piezoelectric pump with flexible valve was proposed for living cells transmission to avoid the cell damage caused by valve material during transmitting process. However, the dynamic switching characteristics of the flexible valve coupled with the flow field directly affected both the flow field inside the pump chamber and the output performance of the pump. At the same time, the output performance of the piezoelectric pump mainly took the optimal working point as a standard. Therefore, the electric-solid-liquid multi-physical field coupling finite element model of the piezoelectric pump with flexible valve based on dynamic mesh was established to study the dynamic characteristics of the valve and the transient flow characteristics of the flow field under the optimal working point. The calculation results of the flow rate show that the output flow rate of the pump is 6.19 g/min at the optimal working point (200 VPP, 10 Hz). The error is 8.94 % compared with the experimental flow rate, which verifies the accuracy of the calculation model. The results of this paper can provide a reference for multi-physical field coupling calculation and optimization design of valve-based piezoelectric pumps with different structures.