The invention discloses a wire arc additive manufacturing method of Al-Mg-Si alloy. The wire arc additive manufacturing method of Al-Mg-Si alloy comprises the following steps that firstly, cooling rolling is used for assisting in arc additive forming; secondly, milling is carried out on the side face and the top face of an additional material body; thirdly, a friction stir machining device is usedfor carrying out friction stir machining on the additional material body, meanwhile, in the friction stir machining process, a cooling rolling device is used for exerting cooling rolling on the sidewall of the additional material body; fourthly, finish milling is carried out on the upper surface of the additional material body for preparing of arc additive forming of the next step; fifthly, theabove steps are circularly and repeatedly executed until the final forming of the part is finished. Dendritic growth and grain refining in the Al-Mg-Si alloy additive forming process can be completelybroken, defects of air holes, cracks and the like can be effectively overcome, meanwhile, in the wire arc additive manufacturing and the modification process thereof, cooling is applied, the problemsof additional material body overheating and microstructure coarsening caused by overheating can be solved, and the mechanical property, particularly, plasticity and fatigue properties, of the addition material body can be greatly improved.