Limit support pressure is a key parameter to keep the stability of the excavation face in shield tunnel excavation, but there has been little research on the limit supporting force of tunnel excavation face for shield machine in upper-hard lower-soft ground． Based on the limit equilibrium method, silo theory and hypothesis of sliding surface being a broken line, the limit supporting force calculation model and formula of excavation face for shield tunnel in upper-hard lower-soft ground were established. To check the rationality of the model and the necessity of considering layers, comparative numerical analyses were conducted. The agreement between the two methods is very good. The effect of buried depth, thickness of upper and lower soil layer, cohesion and internal friction angle on limit support force was analyzed. It shows that the results are different when considering layered or not when the layer is uneven. Therefore, it cannot be equated with homogeneous soil, which should be considered in engineering practice.