Abstract: The isothermal quenching experiment of experimental steel above M_s point was carried. The effect of the introduction of primary marstenite on the dynamics of bestenite phase transition was explored using other experimental methods such as high-resolution dilander, electron backscattering diffraction(EBSD)and X-ray diffraction(XRD), as well as the relationship between the microstructure and mechanical properties after different isothermal time transformations were explored.The results show that the introduction of primary martensite at the isothermal stage below the M_s point significantly accelerates the kinetics of bainite transformation and improves the yield strength and elongation of the experimental steel.This is because the introduced primary martensite provides more nucleation interfaces for the growth of bainite, and obtains a polyphase structure of tempered martensite, secondary martensite, bainite and retained austenite. Among them, a larger amount of retained austenite increase the plasticity of the experimental steel due to the TRIP effect. Finally, after the experimental steel is isothermally held at 300 ℃for 2 h, its tensile strength and yield strength are respectively 1475 MPa and 986MPa, and the elongation is up to 18.5%.