Laser shock peening is a promising surface modification technology to improve the properties of metal. In this work, the spatial distribution model of the shock wave pressure induced by laser shock peening was proposed by matching the pit profile. The relationship between laser beam quality and shock wave pressure distribution was analyzed. And the shock wave pressure profile changes caused by the lateral expansion of plasma were quantitatively calculated. Results showed that the proportions of the flat-topped part and the edge gradient part of the pits formed by different spot sizes are basically the same. As the spot size increases, the range of residual stress increases. Compared with the flat-topped part length of the beam quality profile, the flat-topped part length of the shock wave pressure profile is reduced by about 14% due to lateral expansion of the plasma. Simulated residual stress and deformation profiles are in an agreement with the measured data in all cases.