A valveless synthetic jet-based micropump structure was designed for transporting liquid and a critical structure parameter determination method was developed for the largest flow rate and steady outflow of the micropump. The critical structure parameters included the chamber height and outlet diameter of the micropump. The chamber height could be set at the position where the axial instantaneous velocity in the center axis was invariable. And the outlet diameter could be obtained by the jet boundary on the outlet cross section. A three-dimensional numerical simulation was carried out to obtain the performance of the micropump. The velocity boundary condition and the SST turbulence model were utilized in the numerical simulation. The simulation results of a synthetic jet actuator fit the previous experimental results well. The results suggest that the optimal chamber height and outlet diameter are 7 mm and 1.78 mm respectively. And the flow rate can achieve 32.1 mL/min at the 0 back pressure and Reynolds number of 225. It's effective to design the chamber height and the outlet diameter of the micropump by this method to achieve the maximum flow rate and steady outflow.