Abstract: In the present paper, optical microscope, scanning electron microscope, energy spectrum analysis and brinell hardness tester were used to analyze the defects of welded joint of hadfield steel frogs after dye penetrant inspection. Results showed that the defects on the intermediate welding material and near the fusion line between the welding material and rail steel were caused by the interface cracks between austenite and ferrite or austenite intergranular cracks in the welding material. The main reasons for these intergranular cracks were the fluctuation of chemical composition, the low strength of matrix and the residual stress introduced in the welding process. The defects near the fusion line between intermediate welding material and Hadfield steel mainly located in the side of hadfield steel, which were caused by casting micro shrinkage and phosphorus eutectic. In order to improve the quality of welded joints of Hadfield steel frog, removing the residual stress is one of the methods. More importantly, selecting high-strength single-phase austenitic stainless steel, improving the casting quality of hadfield steel and strictly controlling the content of impurity elements could be key solutions.