Abstract: Taking the commonly used bogie frame material S355 as test material, by adopting laser\|MAG hybrid welding method, the impact of joint gap on back weld formation during backing welding is studied. Through experiments with 6 groups of different joint gap values in the range of 0~1.2 mm, it is concluded that the gap has an evident impact on back weld formation, under the condition of other welding parameters unchanged. The average width of back weld increases first and then decreases with the increase of the gap. The average reinforcement decreases with the increase of the gap. When the gap is 0.4, 0.6 and 0.8 mm, the back welds are well formed, and the ratio of width of back weld and reinforcement is positively correlated with the joint gap. 0~1.2 mm is the applicable joint gap range.