Abstract: In view of the non-linear process of foundation pit floor gushing, a new theoretical analysis method for gushing stability of foundation pit floor was introduced. By simplifying the water-proof layer of foundation pit into a mechanical model of a preloaded fixed-supported beam under uniform load, the total potential energy function of the mechanical structure system was constructed. Based on catastrophe theory, the standard function type and bifurcation set equation of cusp catastrophe model for gushing instability of foundation pit floor were derived, and the failure mechanism of gushing instability of water-proof layer was analyzed. The necessary and sufficient conditions for gushing of water-proof layer were deduced. The results showed that the necessary and sufficient conditions for the gushing of the water-proof layer of foundation pit floor were consistent, which could be used as a judging criterion for the gushing. Through the analysis of an engineering example, it is proved that the evaluation model is scientific, reasonable, accurate and credible. It could provide theoretical basis for preventing the gushing of foundation pit engineering and adopting reasonable supporting measures.