The invention belongs to the technical field of papermaking, and in particular relates to overcoating base paper used for an outdoor highly-strengthened plate and a preparation method of the overcoating base paper. The base paper is prepared from the following raw materials: wood pulp, a filler, an auxiliary agent and a pigment, wherein the auxiliary agent is composed of sodium hydroxide, sulphuric acid, aluminium sulphate and a wet strength agent; and the filler is composed of titanium dioxide, magnesium silicate and kaolin. The prepared overcoating base paper used for the outdoor highly-strengthened plate provided by the invention has a good covering property, fiber distribution uniformity, fine paper surfaces, strong corrosion resistance, a high sun-proof grade, and high water washing color fastness; and after the base paper is impregnated with melamine for veneering, the base paper is impregnated into a solution with the pH of 3.5-10, and no phenomena such as blistering and deformation of the paper happen; and the sun-proof grade is 6 grade or more, and the water washing color fastness is high. The invention also provides a preparation method of the overcoating base paper usedfor the outdoor highly-strengthened plate, and the base paper has high practicability and scientific and reasonable operation, and is simple and practicable.