Abstract: Aiming at the problem of fracture of 40Cr steel automobile half-shafts during assembly, by analyzing the chemical compositions, microstructure, mechanical properties and fracture of the half-shaft, the fracture reasons were found out. The results show that the too high quenching temperature and too long holding time in the heat treatment process led to coarse austenite, and then widmanstatten structure was formed in the center of the half-shaft during cooling, which were the main reasons for the fracture. At the same time, higher quenching temperature led to the aggregation of impurity elements at the grain boundaries and reduced the grain boundary bonding strength, which also accelerated the fracture of the half-shaft. Therefore, optimizing of smelting process system and strictly complying with the heat treatment process system were main measures to avoid fracture of the automobile half-shafts.