The effect of the double reflection layer on performance of thin film bulk acoustic resonator is studied in this paper. The finite element method is used to establish the model of the device with silicon nitride as the support layer, aluminum nitride as the piezoelectric layer and molybdenum as the electrode. Firstly, the influence of the width of the reflection layer on the input impedance is investigated. In order to further verify the simulation results, a cavity structure of thin film bulk acoustic resonator is prepared. The results show that when the width of the reflection layer is 0.5 μm, the quality factor of the device will be greatly enhanced.