Abstract: Welding residual stress is related to the mechanical properties and service life of components. To accurately grasp the residual stress distribution of thick plate 2219 aluminum alloy after friction stir welding (FSW), optimize the welding process, and enhance its service value, a non-destructive testing method—neutron diffraction was employed to measure the three-dimensional residual stress in both FSW single-sided weldments and FSW double-sided weldments with a thickness of 20 mm. Neutron diffraction is currently the only method available for measuring the structural integrity of thick plate materials. And the residual stress distribution of the two welding processes was analyzed and compared. The results indicate that the characteristics of residual stress distribution are similar under both welding processes. And the longitudinal residual stress is the principal stress and manifests as tensile stress. The stress value at the center of the weld is not the maximum residual stress. The significant heat input and large plastic deformation at the weld center lead to stress release in the weld center. The longitudinal residual tensile stress exhibits an asymmetric double-peak characteristic on both sides, with the maximum peak values occurring on both the advancing side and retreating side. And the double peaks of the longitudinal residual stress are located in the shoulder area, where the residual stress drops rapidly to compressive stress outside the shoulder area, approaching the base material zone. The maximum tensile stress of single-sided welding and the double-sided welding are 178.2 MPa and 150.4 MPa respectively. In contrast, because the transverse direction and normal direction are less affected by the temperature gradient, the amplitudes of transverse residual stress and normal residual stress are relatively small, fluctuating around the zero axis. The increase of residual stresses in the longitudinal direction, transverse direction and normal direction almost synchronized, whether in the weld zone or the base material zone. The residual stress distribution in the base material of single-sided welding and double-sided welding also differs. In the base material region of single-sided welding, the longitudinal residual compressive stresses in all three lines are greater than those in double-sided welding. The second pass of double-sided welding is equivalent to a heat treatment process. Double-sided welding exhibits lower residual stresses in all three dimensions compared to single-sided welding. And the double-peak variations of the double-sided welding are more moderate than single-sided welding. The analysis of the test results can provide parameter support and engineering references for the improvement of thick plate welding processes.