Abstract: In design of the bulk acoustic wave (BAW) duplexer, the resonance area of each thin film bulk acoustic wave resonator (FBAR) in Tx or Rx filters affects its performance of insertion loss and out of band rejection. However, these two properties of the filter in design often need to compromise, which brings difficulty for calculating the resonance area of each FBAR in filter. In order to solve this problem, a new parametric design method of Tx or Rx filter is proposed. The circuit model of filter was established by the foundation of FBAR Mason model in ADS software. The resonance area value of series FBAR and the ratio of resonance area value of parallel FBAR to series FBAR were made into two types of optimization parameters reasonably. According to the given insertion loss and out of band rejection of filter as the optimization objective, the optimized values were obtained by the algorithm based on gradient in ADS software. To design BAW duplexer used in LTE band 7 for example, simulation results show that the insertion loss of the Tx and Rx filter using this method is less than 2 dB and the rejection in passband of each other filter is greater than 40 dB, which meets the performance indicators of Tx and Rx filters and verifies the feasibility and ease of use of the method.