Currently there are relatively few studies on the work traits of grille-structured composite foundation composed of cement mixing piles, and its deformation mechanism is not clear. On the basis of field load test results of grille-structured cement mixing pile composite foundation in Xinglong hydropower project, a three-dimensional finite element model which could reflect the mechanics of interaction between soils and piles was established by considering the contact action between soils and piles. The actual loading process of field test of composite foundation’s pile group was simulated using this model. Through comparison of the P-S curve between grille-shaped pile group tests and numerical simulations, the correctness of the numerical model and parameters are verified. Furthermore, the variation regularities of pile’s axial force, vertical stress of soils among piles, and pile’s lateral friction were analyzed. The time course of load sharing relationship between piles and soils is studied, and the distribution of axial force and friction of piles are acquired. According to the vertical balance equation, the average tip resistance and lateral resistance can be calculated by numerical analysis.