The invention provides a method for preparing glyphosate by utilizing visible light to catalyze and oxidize phosphonomethyl iminodiacetic acid. The method comprises the steps of adding the phosphonomethyl iminodiacetic acid as shown in a formula (I) and a photocatalyst to solvent water, stirring for 24 to 48 h under the conditions of oxygen atmosphere, visible light radiation and normal temperature and pressure, and then post-processing reaction liquid, so as to obtain the glyphosate as shown in a formula (II). The preparation method has the characteristics of mild reaction conditions, simplicity in operation, good yield, no pollution, environmental friendliness and the like because the visible light is adopted to catalyze, and the method has a broad prospect.