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