This paper presented a novel piezoelectric micromachined ultrasonic transducer (PMUT) based on mechanical coupling between an annular diaphragm and a circular diaphragm stacking together. The circular diaphragm whose pedestal grounded on the middle of the annular diaphragm, which formed a base excitation system. Compared to non-coupled traditional circular diaphragm PMUTs, the proposed PMUTs achieved higher transmission sensitivity by 173% with the resonant frequency of 6.14 MHz in water, due to the base excitation amplification of the circular diaphragm. Also, the array transmission performance of the proposed PMUT and non-coupled traditional PMUT with circular diaphragm was compared in the same array dimension. The results indicated that the array composed by the proposed PMUT had less cross-talk effect, which kept the central working frequency almost the same when it work solely.