This paper investigates the effect of the protective layer Al2O3 thickness on the acoustic characteristics and device performance in SAW high-temperature sensors with LGS as the piezoelectric substrate and Pt as the electrode using a multi-physics field coupled finite element model. The results show that the vibration displacement of the sound surface wave in the L-wave direction increases with the increase of the thickness of the Al2O3 protective layer. The vibration displacement in the SH and SV wave directions decreases.The diffusion of energy into the interior of the substrate at 6.25% of the normalized thickness of the protective layer. When the normalized layer thickness is 18.75%, the Rayleigh wave disappears and then the body wave BAW mode; increasing the Al2O3 protective layer, the wave velocity v and electromechanical coupling coefficient K2 increase significantly; First-order frequency temperature coefficient τf, 1 and conversion temperature increase with the increase of the protective layer thickness; using Al2O3 thin film for SAW temperature sensor structure optimization, in order to obtain good comprehensive performance normalized thickness should be around 0.94%.