Abstract: 42CrMo steel quenching crack morphology, material chemical composition, microstructure and fracture morphology of the end parts of the lower tie rod were analyzed, and the main causes of quenching cracks in the actual heat treatment process were discussed. The results show that the main reason of cracking is that the cooling rate is too fast and the stress is too large. The rapid cooling rate during quenching is caused by the low concentration of quenching medium or the low temperature of quenching medium. Therefore, controlling the concentration of quenching medium, checking the cooling capacity curve of quenching medium and adjusting the the temperature during quenching can effectively reduce the quenching cracking phenomenon of the end parts of 42CrMo steel lower tie rod, improve the qualified rate of products and ensure the safety and service life of the lower tie rod.