Welding path planning and motion matching of complex tanks are major areas of research for intelligent welding equipment. In the present work, the welding of tank with variable curvature section was taken as the study object, an innovative method to automatically generate the posture of welding torch at the welding spots of variable curvature section was proposed. The torch posture homogeneous transformation matrix was established, and the equation of welding motion matching on basis of the method of torch posture planning was constructed by combining the angle planning equation of the positioner. Then, a set of welding torch posture and positioner’s rotation angles along the welding path of variable curvature section was created in Matlab, and the combination code of the welding torch movement and positioner rotation was obtained to accurately control the welding speed, wire extension and the posture of welding torch during the welding process of the tank. An aluminum alloy tank composed of eight arcs was chosen to verify the proposed technique. The results show that the difference of welding position at the same positioner’s rotation angle ranged from -0.7976 mm to 0.7616 mm, the repeatability accuracy of the posture of the automatic welding system was less than 0.1 mm, and the weld qualification rate of aluminum alloy tank is more than 98% with limited weld defects. That is, the tanks with variable curvature sections can be automatically and accurately welded by the proposed method.