Abstract: First the impact test data of B50A224 steel at a series of temperatures were fitted by different methods,and by comparing it was found that the curves fitted by hyperbolic tangent function method were more in accordance with the changing rules of impact absorbed energy and percentage of brittle fracture surface with the temperature.At the same time,hyperbolic tangent function fitting method could reduce the influence of material nonuniformity on the ductile-brittle transition temperature obtained from the fitting curves.Then the effect of microstructure on the ductile-brittle transition temperature was studied,and reasons for the abnormal data were found out.At last,by improving the heat treatment process,ductile-brittle transition temperature of the steel was reduced and the low temperature impact resistance was improved.