Abstract: Through 4.5 mm groove width completed by 60 passes, 59R2 grooved rails was docked with abrasion resistant steel frogs by the method of ultra-narrow gap welding with constrained arc by flux strips. The welding fixture for special-shaped section docked was designed. The angular distortion was measured during welding, and the largest angular distortion occurred when the thickness of the fourth welding layer was about 13 mm. When the thickness of welding layer was about 128 mm, there was no more obvious angular distortion. Furthermore, the preset anti deformation angle about 1.6°~1.7° before welding could ensure the glancing flatness of the workpiece after welding. The relationship between the root width of sidewall and welding parameters, groove width was analyzed, and the matching of arc voltage and welding current matched to the groove width was given. When the groove width was 4 mm, the appropriate arc voltage was 23 ~23.5 V, and the welding current was 215 ~228 A.