The welding residual stress has different effects on the mechanical properties of aluminum alloy welded joints, such as size stability, fatigue strength and stress corrosion cracking. Therefore, it is very important to evaluate the welding residual stress accurately. In this paper, the residual stress of A7N01 aluminum alloy welded joints was measured by X-ray diffraction. In contrast to the traditional method, the cosα method was used in this paper, the results were compared with those obtained by the conventional sin2ψ method. In addition, the influence of oscillation unit on the test results of the cosα method was studied.