Abstract: An investigation was made to the gas metal arc welding (GMAW)systems, and further for the control purpose a dynamic system and control model was proposed according to the characteristics of the GMAW systems.By means of the feedback linearization procedure of differential geometry, an equivalent, fully controllable and linear model was derived via a homomorphic transformation and a sliding mode controller was constructed for the GMAW system.The simulation results by Matlab showed the effectiveness of the proposed controller and the method is satisfied with exact control of welding arc length.