For shallow tunnel under unsymmetrical pressure with fault, the surrounding rock failure caused by fault diastrophism is a process from anamorphic to mutational with characteristics of discontinuous changes and sharp jump values. Therefore, the surrounding rock failure modes can be analyzed by catastrophe theory. Taking the tunnel portal across fault facture zone of Shanghai-Kunming passenger dedicated line Changsha-Kunming Hunan CKTJ-IX section as the engineering background and according to the surrounding rock instability, the mechanical model for tunnel across the fault is established; the total potential energy function of fault surrounding rock system is built; and the standard type of catastrophe model and the bifurcation set equation are obtained. The time-dependent function of vault settlement deformation is transformed into Taylor series to build the criterion of tunnel surrounding rock stable state for the fusion of catastrophe model and monitoring data. The research results show that the time-dependent function of vault settlement deformation can be converted into the catastrophe model of surrounding rock instability. The surrounding rock instability based on the catastrophe criterion is identical to the actual damage of surrounding rock. The composition of criterion is simple. It has strong practicability for judging the stability of shallow tunnel under unsymmetrical pressure with fault. There is a certain correlation between the value of the criterion and surrounding rock damage degree.