In this paper, the AlN/PZT composite piezoelectric thin film layer SAW device on the monocrystalline silicon substrate was designed, The finite element method was used to study the influences of the thickness of composite piezoelectric material (the PZT thickness hPZT and AlN thickness hAlN) on the phase velocity of zero-order SAW and first-order SAW, electromechanical coupling coefficient and electrode reflection coefficient in AlN/IDT/PZT/Si structure, According to the dispersion characteristics, the optimum film thickness was obtained, The results show that when hPZT=0, 025λ and hAlN =λ in AlN/IDT/PZT/Si structure, the highest phase velocities of the zero-order and first-order SAW are 5 582 m/s and 5 711 m/s respectively, which are suitable for the design of high frequency devices, When hPZT=0, 2λ,hAlN =0, 1λ, the electromechanical coupling coefficient of zero-order SAW wave is the highest of 21, 55%, but the phase velocity is the smallest of only 2 890 m/s, which is suitable for the design of the signal processing devices of wideband low loss surface acoustic wave filter and delay linear structure in the mobile communication field,