In order to study the influence of size effect of square foundation pit on the supporting effect of long and short pile retaining structure, numerical simulation is adopted for analysis of foundation pit of a bridge cap. Firstly, the selected parameters are verified by field measurement and the calculation conditions of different plane sizes are proposed, the size effect is analyzed by the change of stratum stress and soil pressure behind piles. Then, by comparing the deformation and stress of the long-short pile retaining structure and the equal-length pile retaining structure, the influence of the size effect on the supporting effect of the long-short pile retaining structure is examined. The main conclusions arrived at include: When the size of the square foundation pit is small, a continuous and closed “quasi-circular” arching area would be formed at a certain distance from the pit edge, and there is a “quasi-circular hoop effect” similar to the hoop effect in the circular foundation pit. When the arch area is closed as ring hoop, the earth pressure behind the retaining structure pile is significantly reduced. Otherwise, the earth pressure increases, and the increment of the earth pressure behind the pile in the middle of the pit is greater than that at the corner. The supporting effect of the long-short pile retaining structure is less in the square foundation pit with stronger size effect. The size effect of foundation pit is beneficial to its deformation and stability. In this project, the foundation pit with a side length of less than or equal to 14.4 m can make full use of the size effect and use long and short piles instead of equal-length piles for green environmental protection.