The invention discloses an active agent and a method for carrying out surface treatment on materials to be welded of aluminium and aluminium alloys so as to improve the energy utilization rate of the aluminium and the aluminium alloys, deepen the fusion depth of welding seams and further obtain higher welding quality. The active agent for laser welding consists of a chloride and a fluoride, wherein the mass ratio of the chloride to the fluoride is (1-2): (1-4); the chloride is at least one of lithium chloride, sodium chloride, potassium chloride, zinc chloride, magnesium chloride, aluminium chloride, calcium chloride or tin chloride; and the fluoride is at least one of lithium fluoride, sodium fluoride, magnesium fluoride or calcium fluoride. When the surface treatment is carried out, firstly the fluoride and the chloride are mixed fully and uniformly, water is added to form a supersaturated solution, then the supersaturated solution is coated on the surface of a welded test piece as a coating agent, then drying is carried out, and finally laser welding is carried out under the condition of gas protection.