Abstract: A large number of cracks were found under the surfacing layer of a 40CrNiMoA steel rim through the magnetic particle inspection after surfacing welding. By methods of macrographic examination, chemical composition analysis, metallographic examination, hardness test, scanning electron microscope and energy spectrum analysis, the formation causes of cracks under the surfacing layer of the rim were analyzed. The results show that because of the low preheating temperature in the process of surfacing welding, the upper bainites were found in the vicinity of the fusion line, which caused cracks under the action of the shrinkage stress during the welding cooling process.