Fiber laser-metal active-gas welding (MAG) hybrid welding on 6-mm-thick root of X80 pipeline steel is carried out to attain one side welding both sides formation. Mechanical properties of welded joint in optimal parameters such as microhardness, tensile strength, impact toughness, bending strength are studied. Based on this, the effect of thermal cycle caused by hybrid welding on welded joint is investigated. The results indicate that good appearance, defect free joint can be achieved in the optimized parameters, and the cross-section of welded joint exhibits feature of “goblet”, which can be divided into two parts: arc zone and laser zone. The microhardness in arc zone is higher than that in laser zone. Fracture of tensile test occurs in the base metal. The microstructures of weld in arc zone mainly consist of acicular ferrite and a small amount of proeutectoid ferrite. Whereas, The microstructures of weld in laser zone mainly compose of acicular ferrite and bainite-like (upper bainite and granular bainite). The difference in microstructure of heat affected zone (HAZ) between arc zone and laser zone is small, but the coarse zone in arc zone is obviously wider than that in laser zone.