The fast FEM/BEM method that approximates the residual Green’s function by a superposition of leaky wave modes is improved in this paper,and is used to analyze surface acoustic wave(SAW) radio frequency identification(RFID) devices.The improved method distinguishes their contributions from the spatial Green’s function and then contract the coefficients of leaky wave modes, which can avoid Peach’s problem of calculating the coefficients of leaky wave modes by empirical formula.For an example of 128°-YXLiNbO3 substrate,the feasibility of the improved method is confirmed.The improved method is used for the simulation of a SAW RFID device. The simulation result agrees well with the experimental result, which shows that the improved method is accurate.