The present invention relates to a cross-linked porous membrane from hydrolysis of ester-containing side chain and a preparation method thereof. Firstly, membrane material is obtained through copolymerization of four monomers including butyl methacrylate, styrene, sodium sulfonated styrene and vinylbenzyl chloride. In membrane formation, a small amount of lithium chloride micromolecule porogen is added and cross-linked by using tetramethyl hexamethylene diamine to prepare a nanoscale dense membrane through hydrolysis under the alkaline condition. Through the characteristic of hydrolysis of a butyl ester side chain in the polymer under the alkaline condition, the space originally occupied by the butyl ester in the hydrolyzed membrane is vacated; and after hydrolysis, with the appearance of carboxylic acid ionic conduction groups, a large quantity of ester bonds is hydrolyzed, so that the patency of ion transfer channels in the membrane is enhanced. Thus, ionic conductivity of the membrane is greatly enhanced. The nanoscale porous membrane prepared by the present invention not only has good selectivity and battery performance, but also reduces the preparation cost of the membrane to a great extent, and is suitable for application in all vanadium flow batteries.