The thin film bulk acoustic wave device has many advantages of small size and high performance, the related products have been widely used in the mobile communications market. The thickness and material of acoustic layer such as electrode and piezoelectric layers of the film bulk acoustic resonator(FBAR) are main influence factors of resonant frequency. The necessity and principle of frequency trimming for FBAR and the way of scanning etching are analyzed in this paper.The influence of trimming layer film on the freguency of the device at different etching power is studied. The frequency uniformity is increased by 6.5 times, and the frequency deviation is improved significantly by frequency trimming of FBAR.