The invention relates to a purification method for 3, 4, 5-trimethoxybenzoic acid, comprising the following concrete steps that: measured 3, 4, 5-trimethoxybenzoic acid competitive products and sodiumcarbonate solution with the concentration of 45 percent are added into a reaction vessel according to the proportion of 1:4.5, react with each other at the temperature of 90 DEG C for four hours, arecooled to 40 DEG C, have sulphuric acid added in, have pH value regulated to be 1-2, and are dehydrated to obtain 3, 4, 5-trimethoxybenzoic acid fine crystals; 3, 4, 5-trimethoxybenzoic acid fine crystals, activated carbon and water are mixed according to the proportion of 10:2:100, have heat preserved for 30 minutes at the temperature of 85 DEG C, are filtered, have pH value regulated to be 2 through sulphuric acid, are dehydrated and then are dried via vacuum to obtain finished products of 3, 4, 5-trimethoxybenzoic acid. The invention further purifies finished products obtained by the priorprocess, which ensures that the main content of 3, 4, 5-trimethoxybenzoic acid reaches over 99.80 percent and controls total impurities below 0.20 percent.