The stiffener plays an important role in the overall performance of T-stiffened skin assembly which is a primary structure of aircraft. In this paper, a thermo-viscoelastic finite element model that has been verified by experimental results was established to investigate the effect of the stiffener on stress concentration, stress redistribution, and deformation of T-stiffened skin assembly during curing. The size dependence of residual stress and deformation was analyzed by establishing a series of integrated assemblies with different sizes and stacking sequences of the stiffener. The sensitivity of residual stress and deformation to the stiffener size and stacking sequence was investigated. The results reveal that the overall residual stress of T-stiffened skin assembly is mainly sensitive to the fillet radius and the stacking sequence, whereas the residual stress of skin is almost not sensitive to the stiffener size. The maximum deformation of T-stiffened skin assembly is located in the skin, and the evolution of the maximum deformation is significantly affected by the flange length and stacking sequence.