The present invention provides a process for preparing pemetrexed disodium and its intermediate, 4-(4-carbomethoxyphenyl)butanal. The process for preparing the intermediate comprises the following steps: condensing methyl 4-bromobenzoate with 3-buten-1-ol; extracting with an organic solvent during the work-up; adding silica gel to decolorize; and evaporating the organic solvent to give 4-(4-carbomethoxyphenyl)butanal. The product obtained by the present process, with a yield of higher than 80%, and a purity measured by GC of higher than 95%, may be directly used in the next bromination reaction for synthesizing pemetrexed disodium without purification. The present process is suitable for industrial production, as the operation is simple and the reagents used are cheap and readily available.