In traditional calculation method, the critical thick of water-resisting layer at foundation pit can be got by comparing the gravity of water-resisting layer and water pressure of artesian aquifer, which had not considered the influence of shear strength of soil. The result of traditional calculation method is conservative to reality. Based on force equilibrium analysis of water-resisting layer, equations for calculating safety factor of abrupt gush and critical thick of water-resisting layer is suggested in this article, which considered the influence of shear strength of soil. Some cases have been studied with those new equations. It had been found that the result of those new equations is coincident with realities. In order to ensure the security of foundation pit, it is advised that foundation pit can be adjudged safety only both the safety factor of abrupt gush and critical thick of water-resisting layer satisfied requirements.