Two different cooling processes, namely, relaxation-precipitation-control phase transformation+ acceleration cooling (RPC+ACC) and ultra fast cooling+ acceleration cooling (UFC+ACC) were used to produce low welding crack susceptibility steel. The effects of these cooling processes on microstructure and mechanical properties were analyzed by means of microhardness, SEM and TEM. The results show that the yield strength and tensile strength of the RPC+ACC samples were 170 MPa and 65 MPa more than that of the UFC+ACC samples respectively, and kept the same toughness. The main strengthening mechanism of the RPC+ACC process was that lath bainite ferrite was refined by recovery of dislocation and the form of substructure, and fine lath bainite ferrites staggering arrangement and including high density dislocation were obtained by acceleration cooling.