The instability model of tunnel face is established based on silo theory, Terzaghi′s theory and collapse theory of ellipsoid, so as to study the supporting force for maintaining the stability of tunnel face in soft surrounding rock. The formula for calculating the instability of tunnel face is derived and the minimum supporting force of tunnel face and the critical height of complete slump ellipsoid are obtained. And then the values of vertical depth z at different position relationships and soil pressure q0 on ellipsoid surface are given by analyzing four positional relationships between tunnel depth and height of slump ellipsoid. The rationality of the derivation formula and classic formula is verified by comparing. Finally, the ratio of minimum supporting force for maintaining the stability of tunnel face is obtained by a case study. The results indicate that the derivation formula is consistent with numerical simulation.