The work focused on the forming quality of surface-groove backing welds of gas metal arc welding (GMAW). The Box-Behnken design in response surface methodology (RSM) was used to explore the effects of welding voltages, welding currents, welding speeds, and surface radians on the properties of welded joints. The mathematical model was established among process parameters and response indices of welded joints. Experimental results showed that the unmelted gap decreased with the increased welding voltage, welding current, welding speed, and surface radian. Tensile strength increased with the increased welding voltage and welding speed and decreased after increasing with the increased surface radian. Elongation first increased and then decreased with the increased welding voltage, welding speed, and surface radian. Optimal process parameters were obtained by minimizing the unmelted gap and maximizing the tensile strength and elongation: welding voltage = 32 V; welding current = 224 A; welding speed = 32 cm/min; surface radian = 14/36π rad. The errors of the unmelted gap, tensile strength, and elongation were 1.71%, 5.69%, and 1.27%, respectively, by comparing the predicted and actual values. Research results have essential theoretic guidance for improving the penetration and mechanical properties of complex surface grooves welded joints.