Abstract: Fluxes with different formula were coated on 20Mn2 steel pipes surface, and then the steel pipes were welded by fiber laser welding. The effects of flux formula on microstucture and mechanical properties of laser welded joint of 20Mn2 steels pipes by optical microscopy, scanning electron microscopy, energy disperse spectroscopy, micro-hardness tester and tensile tester. The results show that, after coating fluxes, the depth and width of weld seam both increased, grains in weld seam zone all were refined, O and Si elements were found in weld seam, and the hardness of weld seam increased. The optimum formula of the flux was 36.36wt%SiO2, 36.36wt%TiO2, 9.10wt%Cr2O3 and 18.18wt%NaF. When this flux was coated, tensile strength and elongation of the welded joint was respectively 827.9 MPa and 13.9%, microhardness of the weld seam center increased by 21.8% comparing with that of without coating flux.