The Incredible High Performance (I.H.P.) SAW devices have attracted much attention due to their excellent Q characteristics and temperature stability. In order to develop the wideband filter, the design of a 2-layered I.H.P. SAW substrate structure (Cu electrode/5°Y-rotated LiNbO3 piezoelectric layer/SiO2 function layer/Si substrate) is studied in this paper. Considering the mass loading effects, and combining the distribution characteristics of stress and free charge on the interface between the electrode and the piezoelectric layer, the accurate Finite Element Method and Boundary Element Method (FEM/BEM) is used in the study. Using the continuity boundary condition of the multilayer structure, the composite Green’s function and the admittance of the resonator are calculated accurately. The maximum frequency difference between the resonance point and anti-resonance point is sought to optimize the substrate structure size. The calculation results show that the device with the metallization ratio of 0.5 and structure period (λ/2) of 2 μm has a maximum frequency difference of 138 MHz between the resonant frequency point of 880 MHz and the anti-resonant frequency point of 1 018 MHz when the thicknesses of the function layer, piezoelectric layer and the electrode layer are 0.15λ、0.2λ、0.037 5λ, respectively. The conclusion provides guidance for the design of I.H.P. SAW devices for wideband filters.