The invention relates to an electrolyte membrane and application of a composite membrane thereof in a liquid-flow energy storage battery with acidic electrolyte. The electrolyte membrane uses polybenzimidazole or other aromatic heterocyclic polymers as a base membrane or is doped with a strong-acidity solution through composite techniques comprising an organic / inorganic technique, an organic / organic technique, a blending technique, a crosslinking technique, a strengthening technique and the like so that the acid content, the proton conductivity and the mechanical strength of the composite membrane are improved. Different from the traditional electrolyte membrane containing functional groups, the novel electrolyte membrane does not contain iron conduction groups, but utilizes acid in the electrolyte as the electrolyte to conduct protons. Due to the adoption of a Hopping conduction mechanism, the membrane reduces the iron permeation and the water migration of the liquid-flow energy storage battery, greatly reduces the crossing of anode and cathode electrolyte and prolongs the service life of the electrolyte. The preparation process is simple, and the prepared electrolyte membrane can effectively satisfy the requirements on the performance and the stability of the liquid-flow energy storage battery.