The robot MAG welding method is used for flat welding test,and the influence of the important welding parameters on the appearance of weld in the rapid metal forming process of robot MAG welding is studied. By single factor test method,the influence of welding current,welding voltage,welding speed and wire extension on the appearance of weld are tested respectively,and the influence rules of different welding parameters on weld geometry dimension（weld width,reinforcement and penetration） are obtained. The experimental results show that welding current,welding voltage and welding speed have significant influence on the weld geometric dimension,while wire extension has little influence on the weld geometric dimension. The weld geometric dimension can be adjusted slightly by changing the welding parameters.