In this paper, a threedimensional (3D) mechanical analysis is conducted for soft soil cutting by shield machine, on the basis of Rankine’s Earth Pressure Theory and MckyesAli cutter resistance model. In this modified MckyesAli model, the cohesion of soils, normal stress imposed by cutter and ground, deadweight of soil, parameter L and parameter Ψ are taken into account in association with force balance theory. It shows that the modified model is suitable for analysis on stress of any point of the cutter on cutterhead. The modified model is then compared with existing models, and the applicability of the model is verified by using Excel computing interface. The paper can provide suitable formula for the feasibility design of the cutter of shield machine.