This paper presents a new 3D collapse mechanism to investigate the face stability of a shallow tunnel embedded in the saturated and multi-layered soils with short-term condition. The new collapse mechanism is composed of five rigid truncated elliptical cylinders on which a distributed force acts. The limit support pressure is determined by an upper-bound analysis. A simple method given by the fitting formula is presented to estimate the limit support pressure. Analytical solutions obtained by the new collapse mechanism are validated by numerical simulations. It has shown that the proposed collapse mechanism has good accuracy and efficiency.