The Complex reheating phenomenon during friction stir additive manufacturing (FSAM) has a significant impact on the microstructural evolution. This, in turn, affects its mechanical properties. A flow stress model including the precipitate, solid solution and dislocation density evolution was proposed to reveal the relationship between the microstructure and constitutive behavior in FSAM of Al-Mg-Si alloys. The microstructure and mechanical properties of single-layer and multi-layer FSAM were investigated using experimental and numerical simulation methods. The results revealed that during the first reheating process, the precipitates exhibited dissolution and coarsening behavior in the heating stage. In the third reheating process, precipitates were generated during the heating stage because of the lower temperature. The multiple reheating process in FSAM promoted the gener- ation of precipitates in the stirring zone. This phenomenon increased the yield strength from 183.46 MPa to 189.95 MPa. Meanwhile, the precipitate nucleation and growth during reheating process depleted the concen- trations of Si and Mg in the matrix. A comparison of the stress-strain curves before and after the reheating process, revealed that the reheating process reduces the net flow stress in the plastic deformation stage. A decrease in the concentration of solid solution elements caused a decrease in the statistically stored dislocation density, and thereby, decreased the net flow stress.