Abstract: In-service welding is one of the most important orientations of repairs of oil and gas pipeline. This paper presents the research progress on in-service welding technique of oil and gas pipeline in China and abroad from two aspects of influencing factors and process assessment. Based the failure cases of in-service welded joints and relevant studies in China and abroad, this paper analyzes the main factors influencing the in-service welding. It is pointed out that, in addition to burning through and hydrogen-induced cracking, medium thermal decomposition, pipe wall carburization, hydrogen permeation, stress corrosion and fatigue should be considered in welding assessment. Some problems in practical in-service welding operations, like preheating and difficulty in reaching the standards of interlayer temperature, and related solutions are proposed.