Abstract: The small-scale model test is an important method to study the bearing capacity in soil mechanics. However, the size effects of small-scale model will affect its quantitative results, so that the small-scale test can only be used for qualitative researches. In this study, 20 groups of small-scale model tests on bearing capacity of strip foundation are carried out in saturated sand foundation. The change of bearing capacity is studied by changing the foundation size, buried depth and compactness of sand. Then the discrete element analysis method is introduced, and the mechanical behavior of sand particles is simulated by rolling the resistance contact model. The bearing capacity tests on the strip foundation are simulated, and the source of the size effects of small-scale model tests and their influences on macroscopic bearing capacity are revealed from a microscopic point of view. The results show that there is a size effect in the small-scale model tests on the saturated sand foundation. The larger the foundation compactness and foundation depth, the more obvious the size effects. The size effects are the result of the combined effects of change of stress level, change of failure mode and degree of progressive failure.