Abstract: The thick plate of high strength steel S420 used for deep-water jacket is prone to generate transverse crack during the actual welding process. The transverse crack sensitivity of weld metal under high constraint and high diffusion hydrogen content was tested through G-BOP tests in a low-temperature and high humidity environment. The influence of different heat input on the microstructure and transverse cracks of the weld was studied. The experimental results show that the sensitivity of transverse cracks in the weld seam is highest under low heat input. When the heat input is moderate, transverse cracks are less likely to occur. However, transverse cracks also occur under high heat input, but their tendency is smaller than that under low heat input. When it is welded in low temperature and high humidity environments, the diffusion hydrogen content slightly decreases with the increase of the heat input, but the diffusion hydrogen contents of weld metals with different heat inputs are all high, and there is no linear correlation between the crack rate of transverse cracks and the diffusion hydrogen content. The weld under low heat input is prone to produce transverse cracks mainly due to the formation of a large number of feather like upper bainite, and the cracks tend to germinate in bainite and expand along the direction of bainite lath/ lath bundle. The transverse cracks in welds with high heat input tend to initiate and propagate in proeutectoid ferrite. The coarse columnar proeutectoid ferrite and the heterogeneity of proeutectoid ferrite/acicular ferrite structures are the main reasons for the transverse cracks.