This paper investigated the welding process of one-side welding with backing formation of 6 mm 2A14 -T6 aluminum alloy, and analyzed the discipline of the alternating current frequency and assembly accuracy to the welding formation and the joint performance. The results showed that with the AC frequency gradually increased from 60 Hz to 80 Hz, the average tensile strength decreased from 356 MPa to 310 MPa, but the joint elongation did not change significantly. The analysis of assembly accuracy showed that the body material would be burnt through or single cutting by one-side when the assembly clearance exceeded 20% of thickness which was unable to form welding leakage. On the other way, when the larger assembly stagger was come out, the mechanical properties of joint decreased obviously. In addition, the influence of stagger on the performance was greater than the clearance. When the stagger was increased, the backing formation would be worse. The result of joint performance showed that if the stagger was greater, the joint tensile strength and elongation declined gradually. So in practical engineering applications, the local stagger of joints should be controlled within 25% of thickness.