Advanced high-strength steel (AHSS) is increasingly used in the automotive industry due to its higher strength and lower weight. The traditional forming limit criterion cannot accurately predict the unique shear fracture of AHSS, so great efforts have been made to develop failure criteria that can predict shear fracture. In this paper, a series of tensile and shear tests for four steel sheets of AHSS are designed, the stress triaxiality and equivalent strain to fracture are solved, and the correlation between them and the performance parameters of steel sheets K and n is studied. In order to study the relationship between stress triaxiality and equivalent strain to fracture in the range of low-stress triaxiality, the Hill'48 orthotropic model and modified Mohr–Coulomb (MMC) fracture model were used to establish tensile and shear fracture models of four dual-phase sheets of steel, simulating and studying the plastic fracture of AHSS. Solving the relevant parameters enriches the stress triaxiality of the four steel types, establishes the relationship between the stress triaxiality and the equivalent strain to fracture, and verifies its correctness through tensile and bending tests and simulations. The results show MMC can accurately predict the fracture of these four dual-phase steels, and the quantitative relationship between stress triaxiality and equivalent strain to fracture of the four dual-phase steels in the low-stress triaxiality range 0–0.3 is similar, which can be established and expressed by the performance parameters of each steel type.