Abstract: Welding current and arc length will determine the metal transfer,heat input and weld appearance for gas metal arc welding (GMAW).The mathematical model of welding current and arc length is present through the analysis of GMAW welding process,a direct model reference adaptive control based on the quadratic performance index for the GMAW process is designed,and welding current and arc length are controlled by adjusting of welding power source voltage and wire feed speed to trace the output of the reference model.At the same time,an estimation model is given for indirect measurement of the arc length,which is difficult to detect in the application.The results of simulation show the validity of the control method.