Double–wires+arc additive manufacturing (D–WAAM) system for Al–Mg–Si alloy using hybrid pulsedvariable polarity gas tungsten arc welding (HPVP–GTAW) process was established. When two different alloying wires (Al–5Mg + Al–5Si) were used, ternary (Al–Mg–Si) alloys with various compositions can be obtained by adjusting the wire feed speed of the two wires. Mechanical properties of the as-deposited and heat treatment Al–Mg–Si alloys were investigated. Microstructures of Al–Mg–Si deposits are mainly composed of columnar and equiaxed dendrite grains with non-uniform distribution characteristics. Mechanical properties of the as-deposited and heat treatment Al–Mg–Si alloys were investigated. Experimental results show that with the increase of the Mg/Si ratio, tensile properties of the as-deposited alloy increased, which were also nearly isotropic along the two directions. After heat treatment, tensile property of some specimens increased a lot but too much plasticity was loosed.