Abstract: The delayed crack of high strength steel S420 in deep-water jacket is very harmful to the strength and safety of jacket structure because of its concealment. Therefore, it is necessary to strictly control the weld quality of jacket structure. The diffusion hydrogen content in weld metal is one of the main causes of delayed cracks. In order to study the influence of construction environment factors (ambient temperature, humidity, wind speed and welding material exposure time) and process parameters (welding heat input and preheating temperature) on the diffusion hydrogen content of weld metal during the welding process of high strength steel S420 for deep-water jacket, this paper used flux-cored wire gas shielded welding (FCAW-G) and submerged arc welding (SAW) processes to test the diffusion hydrogen content of high strength steel S420 in different simulated environments. The internal relationship between the source of diffusion hydrogen during construction and the actual construction combination conditions and the diffusion hydrogen content of weld metal was studied. By analyzing the source and key influencing factors of diffusion hydrogen in the weld metal of high strength steel S420 for deep-water jacket, effective preventive measures were proposed from the perspective of reducing diffusion hydrogen content to reduce the probability of delayed cracks in deep-water jacket welding.