Abstract: Mechanism of superplasticity of high-strength low-alloy quenched and tempered steel in the two-phase region is proposed through analysis of tensile stress-strain characteristics of the 10CrNi3MoV steel at superplastic temperature.Microstructure and fracture behavior of different degrees of deformation and numerical simulation of the distribution of plastic strain fields were analyzed.Results showed that the austenite phase with low yield stress around the ferrite phase with high yield stress is unicom,and it acts as a "lubricant" role through its own plastic deformation.After a certain degree of deformation,stress of the austenite phase exceeds the yield stress of ferrite phase because of strong strain hardening,which promotes the plastic deformation of the ferrite phase.Ferrite phase in high strain zones transforms into austenite grain,which supplies lubricant for the subsequent deformation.Continuous plastic deformation- phase transition maintains the large deformation behavior overall and continuity,material shows superplasticity.