Abstract: The effect of normalizing + tempering and isothermal heat treatment processes on the microstructure and mechanical properties of U20Mn2SiCrNiMo bainite steel rail has been studied. The results show that as the test steel is normalized at 900 °C + tempered at 300 °C ,the mechanical properties are tensile strength 1396 MPa,elongation 16.0% impact absorption work K_U2 57 J,and HB hardness value 402；as the test steel is heated at 870 ~ 930 °C after air-cooied to 300 °C isothermal treatment,the mechanical properties are tensile strength 300 MPa,elongation 17.0% ,impact absorption work K_U2 ≥80 J,HB hardness value 375 ~ 395 ；By comparison with the traditional normalizing + tempering process, the optimized isothermal heat treatment process can greatly improve the impact toughness of U20Mn2SiCrNiMo bainite steel rails i. e. the room temperature impact absorption work has been increased from 57 J to more than 80 J ,by 40% ~ 56% , while the elongation after breaking remains basically unchanged, tensile strength and tread hardness slightly lower. The best optimized process is normalizing at 870 °C. , air-cooling to 300°C for 4 hours air-cooling.