The invention relates to a preparation method of a sun-screening agent OMC (iso-octyl P-Methoxycinnamate). The preparation method comprises the following steps of: 1, adding p-anisaldehyde, malonic acid, a catalyst I organic secondary amine substance, a catalyst II organic acid substance and a water-insoluble organic aromatic hydrocarbon solvent to a condensation reaction kettle, and carrying out reflux water diversion reaction for 4-5 hours; 2, after condensation reaction is finished, cooling to 5-15 DEG C, filtering to obtain a p-methoxycinnamic acid crude product, directly applying filter liquor to next reaction, and optionally supplementing a small quantity of catalysts and solvents to the filter liquor; 3, adding catalyst III macroporous type storng acid cation exchange resin, isooctyl alcohol, the p-methoxycinnamic acid crude product and an aromatic hydrocarbon or aliphatic hydrocarbon solvent to an esterifying reaction kettle, and carrying out reflux water diversion reaction for 2-3 hours; and 4, after esterifying reaction is finished, cooling to 65 DEG C, filtering and recovering the catalysts for mechanical application, washing the filter liquor to neutrality through a 5% sodium carbonate water solution, desolventizing, and rectifying to obtain the final product isooctyl p-methoxycinnamate. The method disclosed by the invention has the characteristics of easiness, convenience, easiness in control, industrial production, low environment pollution, low energy consumption, material recycling, high product purity, and the like.