This paper presents an equivalent circuit model, a systematic design, and optimization method for developing a broadband annular diaphragm piezoelectric micromachined ultrasonic transducer (A-PMUT). By utilizing array analysis methods, an annular diaphragm is regarded as an array consisting of equally spaced sector diaphragms influencing each other by crosstalk effect. The model successfully explains the phenomenon of multi-resonance peaks in the frequency response curve, sharing the same vibration mode. The study finds that the analytical predictions of the model are in good agreement with the simulation and experimental results. Meanwhile, based on the phenomenon of multi-resonance peaks, a systematic design method is proposed to extend the bandwidth of the A-PMUT. In this method, the radiation impedance of the A-PMUT is separated into crosstalk-free and crosstalk contributed parts. This method enables the determination of the optimal structure counting for the influences on the frequency response of A-PMUTs for broadband applications. The model here can also be further generalized to be a guideline for the design and optimization of broadband PMUTs.