On the basis of the spatially mobilized plane (SMP) theory, a theoretical solution for the spherical cavity expansion pressure of belled piles is derived. Energy dissipation principles are applied to analyze the whole process of spherical cavity expansion. A yield criterion of spherical cavity expansion is obtained by using stress invariant. Then an elastic-plastic zone stress expression is developed by simplifying differential equation to derive the displacement and strain expressions. The solutions of the expansion pressure are derived by the volume conservation and energy conservation separately. The elastic deformation of the plastic zone is considered and the relation among the pressure p, plastic zone radius R and the expansion radius is established. The analyses demonstrate that the calculated value of expansion pressure agrees well with field testing results. The results also show that the plastic zone radius and expansion pressure increase with the increase of the expansion radius, but the growth rate gradually slows down and tends to be stable. The dilatancy angle of soil has significant influence on the plastic radius and expansion pressure. The plastic zone radius and expansion pressure increase greatly with the increase of dilitancy angle.