The invention relates to an intermediate of pemetrexed disodium, a preparation method thereof and a method for preparing pemetrexed disodium thereby; and the intermediate is (2-(4-(3-(2,4-diamino-6-oxy-1,6-dihydro-pyridine-5-group)-3-(1,3)dioxolane-2-group-propyl) benzylamine)sodium glutaric acid. The synthesis of the intermediate comprises the following steps: firstly, condensation reaction is conducted on 4-bromobenzoic acid or 4-iodobenzoic acid and L-glutamate diethylester, then Hack reaction is conducted, 4-bromo is replaced and 4-butyraldehyde is formed, then selective bromo replacementis conducted and the 4-butyladehyde is converted into 2-bromobutyraldehyde, and then condensation reaction of aldehyde and ethylene glycol is utilized for protecting the aldehyde, and pyrimidine ringis further synthesized, and finally the intermediate is obtained. Acid hydrolysis ring-closing reaction and sodium hydroxide salification are respectively conducted for once on the intermediate so asto obtain the pemetrexed disodium. The method for preparing pemetrexed disodium in the invention has high yield, low cost and easy operation and is applicable to industrialized production.