The application of weathering steel sheet is the core technology to solve the lightweight of high speed heavy load truck. In order to solve the problems of large heat input,burn through and large deformation during the welding process of weathering steel sheet,the CMT welding method is used to optimize the welding parameters of 1.5 mm weathering steel sheet,and the optimal parameters are used to simulate and compare the deformation of CMT welding with that of traditional welding（MAG welding）for weathering steel. The results show that the properties of the full penetration joints are good,and increasing welding speed can reduce joint misalignment and improve the joint property,the tensile strength of joint peaks at 669.6 MPa;the simulation results show that the deformation trend of CMT welding is similar to that of MAG welding,the deformation of weld center and edge plate are large,the maximum CMT welding deformation is +3.8 mm,the maximum MAG welding deformation is +4.8 mm,the CMT welding deformation is obviously smaller than MAG welding deformation.