Due to the fact that the analytical method of foundation additional stress is the key of foundation stability analysis, the mechanical analysis of foundation additional stress based on semi-infinite continuous linear elastomer theory is widely used in engineering currently. However, the soil is not continuous line-elastic materials, but granular materials with particles of different sizes and disorderly arrangement. Therefore, with the characteristics of foundation soil bulk medium taken into consideration, an improved calculation method of foundation additional stress under vertical rectangular uniformly distributed load has thus been established, with its rationality to be verified. On this basis, by adopting the orthogonal experimental design method, an analysis has been made of the effects of vertical uniformly distributed load, internal friction angle and particle non-uniformity coefficient on the additional stress of foundation. The results show that the factors affecting the additional stress include the load size, particle non-uniformity coefficient and internal friction angle in a descending order. What’s more, the additional stress decreases with the increase of particle non-uniformity coefficient and internal friction angle, while increases positively with the applied load, with its increment decreasing gradually with the increase of load.