In this study, the effect of the electrode-boundary step structure on the spurious mode of thin-film bulk acoustic resonators (FBARs) is investigated. The finite-element simulation method is used to investigate the effect of the width dimension of the step structure on the suppression of spurious modes. The step structure is analyzed in conjunction with the vibration pattern to suppress the leakage of acoustic energy and to improve the quality factor of the device. To further verify the simulation results, FBAR devices are experimentally prepared. The results show that when the width of the raised frame is 3 μm and the width of the recessed frame is 1.5 μm, the resonator spurious mode is effectively suppressed and the quality factor at the anti-resonance frequency is increased by about 100.