Abstract: During the actual welding of the high-strength steel S420 for deep-water jacket, the flux cored welding wire is often exposed to the humid air environment, resulting in an increase in the content of diffusing hydrogen, and the weld metal is easy to occur delayed cracks. The influence of different preheat temperature on delayed crack sensitivity of weld metal is studied by diffusing hydrogen test and oblique Y-groove test, then the microstructure characteristics, hardness and crack initiation/growth behavior are analyzed. The results show that the diffusive hydrogen content increases from 4.0 ml/100g to 7.0 ml/100g with the exposure of welding materials in wet environment. There is no delayed crack on the weld, when the welding material is not exposed and the preheat temperature is 130 ℃. The crack rate of weld metal is up to 25%, after the welding material is exposed for 24h and the preheat temperature is 130 ℃. When the preheat temperature increases to 150 ℃, there is no delayed crack in the weld. In order to effectively prevent welding delay crack of high-strength steel S420 of large and thick plate, increase of diffusing hydrogen content caused by exposure of welding material in welding environment shall be considered when setting preheating temperature, and preheating temperature shall be properly increased.