We present simulation and experimental studies of a piezoelectrically actuated microdiaphragm air pump, which is characterized by thin structure, large flow, and low power consumption. A novel large-displacement actuation structure is designed for the air pump. A prototype of the micro air pump is fabricated by precise fabrication. Furthermore, studies of modeling, simulation, and experiments are carried out. The experimental and simulation results demonstrate that both the pump's flow and the amplitude of the actuation structure's motion depend on the frequency of the input voltage. The maximal values of the flow and the amplitude will be obtained when the frequency of the input voltage is equal to the actuation structure's first-order natural frequency. The diaphragm air pump has the best performance when it works in resonance mode. With 20-V input, the flow and the amplitude of the air pump are 4.5 ml/s and 0.00041 m, respectively, and the power consumption of the pump can be as low as 3.18 mW. With the advantages of large flow, thin structure, and low power consumption, the diaphragm air pump has great potential applications for air supply for microfuel cells or the cooling of electronic devices.