Abstract: To make a new high strength steel be better applied in impacting environment, an isothermal salt bath method was proposed for its heat treatment. The microstructure of the material was observed by OM,SEM and TEM. The quasi-static tensile properties were tested by a universal testing machine and the dynamic properties were tested by split Hopkinson pressure bar (SHPB). Meanwhile the nucleation of adiabatic shear bands (ASBs) under critical strain rates was obtained. Experimental results show that, with the increasing of isothermal temperature, the main microstructure is changed from martensite+lower bainite, martensite+lower bainite+upper bainite to martensite+upper bainite, and the yield stresses and toughnesses decline gradually. The critical strain rate for the nucleation of ASBs of the steel treated at 330℃ isothermal temperature is the lowest. The appearance of upper bainite makes the high strength steel less sensitive to the nucleation of adiabatic shear bandings.