The invention discloses a long-ageing reusable plating assistant agent for zinc-aluminum-magnesium alloy for hot dipping. The plating assistant agent is prepared from the following raw materials in percentage by weight: 25 percent to 35 percent of ZnCl2, 2 percent to 10 percent of KF, 5 percent to 15 percent of CaCl2, 4.5 percent to 8.5 percent of AgCl, 2 percent to 3.5 percent of alkali-free SP binary compound flooding dispersing agent, 0.1 percent to 0.5 percent of compound surface active agent and the balance water. Through a reasonable formula of the plating assistant agent, a zinc-aluminum-magnesium alloy coating is obtained on the surface of a steel and iron material substrate by utilizing hot dipping; the coating and the steel and iron material substrate separately keep original performance; the coating is compact in structure and free of skip plating, and zinc-aluminum-magnesium ternary eutectic with excellent corrosion resistance can be formed. The plating assistant agent is low in cost, can be easily produced industrially, and can be widely suitable for hot dipping long-ageing reusable zinc-aluminum-magnesium alloy plating solution of which the Al content is 5 percent to 22 percent and the Mg content is 2 percent to 6 percent.