Assessment of the residual stress-strain state (SSS) at MIG welding of aluminium alloys structures is relevant, because of the need to predict formation of residual stresses and strains, which may lead to lowering of fatigue strength of such a structure. Therefore, the research is devoted to determination of the influence of the mode of pulse-arc (MIG) welding of 1561 aluminium alloy of Al-Mg-Mn system (4 mm thickness) on the thermal cycle of the process and prediction of the influence of rigid restraint of the welded specimens in the assembly-welding fixture on the residual SSS of butt and tee welded joints. For this purpose, a finite element model of calculation of the temperature fields and SSS was created on the base of the conduced technological studies, which ensured up to 10% accuracy, that is an acceptable result for technological calculations. This model was used to determine the longitudinal and transverse tensile stresses and displacements, equivalent strains and stresses, out-of-plane displacements of the welded joint of Al-Mg-Mn aluminium alloy. It was found that rigid restraint of the specimens leads to formation of maximal equivalent stresses at MIG welding: for the butt joint - up to 190 MPa, forming in the points of transition from the weld to the base metal on the face surface; for the tee joint - ~250-260 MPa, forming in the weld area in the flange. Out-of-plane displacement after unfastening of the welded specimens from the assembly-welding fixture was equal to: for butt joint – up to 0.1 mm; for tee joint – up to formation of a plastic zone in the flange ~1.7 times larger than in the web, and out-of-plane bending of the web by 1.31 mm.