The stability of the up-stream lock head of Three Gorges ship lift is analyzed with elasto-plastic limit equilibrium method in which the critical instability state is defined as the state where the plastic zone runs through the rock foundation

and the pre-determined sliding surface is unnecessary. The limit equilibrium state is approximated stepwise with decrement of material strength. The construction sequence is taken into account in the study

and the stresses of rock after excavation are treated as initial stresses when reloaded . And because of the consolidation grouting in rock after excavation

rock reloading begins in elastic state. Results show that the safety factor of stability of the up-stream lock head is 3. 3.