Abstract: The effects of elements content, groove shape and welding heat input on the welded joint microstructure transformation, impact toughness, heat affected zone softening degree of EQ51 ocean engineering steel plate were studied by instrumented charpy impact test, EBSD and SEM. The results indicated that C and Mn contents in base metal play a key role on weld joint microstructure type and orientation. When C and Mn contents reduce respectively from 0.1%, 1.5% to 0.06%, 1.3%, welded joint microstructures transfer from lath bainite to acicular ferrite, crack arrest ability obviously increases and its impact absorption energy exceeds 100 J. Appropriately increasing heat input would improve the harmful effect of quench hardening elements on welded joint toughness. Ni and Mo don't affect microstructure transformation of welded joint obviously, but play a critical role on heat affected zone softening degree. Total content of Ni and Mo reduces from 0.7% to 0.4%, softening zone width increases from 1 mm to 2 mm and tensile strength decreases from 789 MPa to 650 MPa.