Abstract: For high hardness problem of joints in local dry underwater welding, arc preheating technology was proposed to improve local dry underwater welding quality according to the technical characteristics of the fixed drainage cover. Not only the design and installation of heating plate but also arc preheating operation mode were illustrated. Then the feasibility of arc preheating was verified by experiments. Process tests of weld joints with preheating and no preheating were carried out, respectively, and the effect laws of arc preheating on the comprehensive physical property of weld joint were summarized. The results show that the maximum hardness of heat affected zone of weld joint is obviously reduced, impact energy is significantly increased and comprehensive physical property of weld joint is effectively improved when arc preheating is applied in local dry underwater welding.