The invention discloses a preparation method of trityl olmesartan. The method comprises the following steps: 1) trityl olmesartan ethyl ester is hydrolyzed by inorganic alkali in the mixed solvent of tetrahydrofuran and low-level alcohol under the anhydrous condition to obtain trityl olmesartan salt; 2) the trityl olmesartan salt is added with acid for acidifying and crystallizing to obtain the trityl olmesartan. As the preparation method adopts the tetrahydrofuran and the low-level alcohol such as methanol as the solvent, side reactions are few under the anhydrous condition, the purity of the obtained trityl olmesartan target product is high, and the yield is ideal.