In elastic-plastic solution of expansion of spherical cavities, based on Mohr-Coulomb strength criterion, the effect of intermediate principal stress on yield and failure of soil is not analyzed. Therefore, there is disparity between results obtained by Mohr-Coulomb strength criterion and those obtained from experimental data. In this paper, the elastic-plastic solution of expansion of spherical cavities, based on unified strength theory is established and unified solutions are obtained. The corresponding solutions of different materials can be obtained. The unified solution cannot only be used to fit the properties of the materials with different tension-pressure strength, but also those with equal tension-pressure strength. The results show that by this solution, full use of the properties of the materials can be attained to reduce supports, which is of important significance for engineering