Static stress of granular materials stored in silos is important for the security of materials during storage. Janssen equation was commonly used to calculate the static stress of grains in silos, but later researchers found the equation underestimated loads in storage bins. In this paper, the modified Cam-clay model was applied to study the stress distribution of wheat piles at four moisture levels, stored in a circular flat bottom silo, at different grain depth and radius of silos. The results showed that as the grains depth increased, the vertical stress of wheat piles increased except the wall-bottom of silos, and the lateral stress increased except the center of silos near bin bottom. Moreover, it was found that as the radius increased, the vertical stress of wheat piles decreased at the same depth and moisture content, and the lateral stress decreased but increased near the bin bottom in the same case. When the mean stress of a layer of grains were observed, it was found that the lateral stress was negatively correlated with moisture content when the vertical stress had no concern with it at the same grain depth and all of them approached the maximum at the same moisture content.