Abstract: The effects of the heat treatment process of isothermal and tempering on the structure and properties of B/M multiphase steel. Jmatpro software is used to simulate temperature-phase equilibrium diagram and TTT curve. Microstructure and mechanical properties are tested and analyzed by JSM-5610LV scanning electron microscope, JB-300B impact testing machine, HRS-150 digital Rockwell hardness tester etc. Results show in the experimental conditions, the ideal heat treatment process parameters are as follows: the quenching temperature is 870 ℃, the temperature of leaving liquid is 318 ℃, the isothermal temperature is 370 ℃. Matrix microstructures of the material are the combination of lower bainite and martensite. Impact toughness of the material is in the range of 28 to 30 J/cm². Hardness is in the range of 45 to 47 HRC, the hard toughness is improved obviously.