Abstract: Based on complex single piezoelectric layer's film bulk acoustic resonator (FBAR), a new kind of complex double piezoelectric layers' FBAR is proposed. This structure can greatly improves the flexibility of the selection of piezoelectric materials for FBAR. The analytical expression of input impedance is derived and simulated. The results of simulation show that: 1) the resonant frequency of basic mode increases significantly with the thickness ratio of the piezoelectric layer with higher acoustic speed to the whole thickness of double piezoelectric layers, 2) the relative bandwidth increases with the thickness ratio of the piezoelectric layer with higher electromechanical coupling coefficient to that with lower electromechanical coupling coefficient, and 3) some modes can be found in double piezoelectric layers' FBAR but not in the single layer's FBAR.