In order to study the influence of the size effect of square foundation pit on the supporting effect of long and short pile retaining structure, based on the foundation pit of a bridge cap, the numerical analysis method is adopted. Firstly, the selected parameters are verified by field measurement. Based on this, the size of different foundation pits is determined, and 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 proved. The main conclusions are as follows : (1) When the size of the square foundation pit is small, a continuous and closed “quasi-circular” arch area will 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. (2) When the arch area is closed as ring hoop, the earth pressure behind the retaining structure pile decreases significantly. When it is not closed, the earth pressure increases, and the pressure behind the pile in the middle of the pit edge increases the earliest and is greater than that at the corner of the pit. (3) The supporting effect of long and 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. Small size foundation pit should fully consider this effect and adopt long and short pile retaining structure to achieve green environmental protection.