Abstract: The synthesis of limettin is studied. 5,7-dihydroxycoumarin was synthesized through phloroglucin and ethyl propionate by the catalysis of ZnCl₂, with a yield of 97.7%. 5,7-dihydroxycoumarin was reacted with methylation reagents giving limettin. The conditions of methylation reaction were studied in detail and the optimization conditions determined with a yield of 75.7%: dimethyl sulfate was used as the methylation reagent, while dimethyl sulfate and 30% NaOH solution was dropped in alternatively. The mechanism of methylation reaction is discussed.