Abstract: Process research on 12.5 mm thick low alloy high strength TATM700 girder steel for automobile structural parts was studied by plasma-MIG hybrid welding equipment. The weld formation, weld microstructure and mechanical properties under different groove forms were analyzed. Furthermore, it compared with the multi-layer manual TIG method of flux cored wire. The results showed that the groove form has less influence on the plasma-MIG composite welding seam. The sample welded by the composite welding method is well formed and the cross-sectional shape is reasonable. The weld structure consists of a small amount of side slab ferrite and a large number of acicular ferrite. The joint strength and bending performance are better than manual TIG welding, the tensile strength reaches 95% of the base metal, and the hardness is slightly lower than the multi-layer multi-channel hand-welded TIG welding sample.