Abstract: The 18MnD5 low alloy base metal and E309L stainless steel surfacing layer cracked along the fusion line after dissimilar steel welding of a safety end. The cracking causes of the fusion line after welding were analyzed by means of macro analysis, chemical composition analysis, mechanical property test, metallographic examination and micro analysis. The results show that due to the small welding heat input, insufficient bath temperature and fast cooling rate, the martensite hardening structure was produced, which made the stress concentration at the fusion line and caused the cracking between the 18MnD5 low alloy base metal and the E309L stainless steel surfacing layer of the safety end. At the same time, the main crack caused the secondary intergranular cracking of the stainless steel weld, and the rapid cooling rate accelerated the initiation and propagation of the secondary crack, thus accelerated the cracking of the fusion line at the safety end.