The invention relates to a preparation method of a composite membrane, and belongs to the field of ionic membrane preparation. The preparation method comprises the following steps: by taking perfluorinated sulfonic acid resin with good chemical stability as a membrane material main body, mixing polymers with different functions, dissolving the resin through an organic solvent to obtain slurry, adding an additive, and uniformly mixing to obtain a gel material; putting the gel material into an extruder for extrusion molding, allowing the molded membrane to enter environments of different temperature sections, gradually reducing the different temperature sections from high to low, allowing the membrane subjected to gradient heat treatment to enter an extraction agent for extraction, taking out the membrane, allowing the membrane to enter deionized water for washing and for calendering and rolling through a hot roller to obtain the porous ion exchange composite membrane. The continuous preparation process of the composite membrane is simple, and the prepared membrane is low in swelling, good in size stability, high in mechanical strength, low in vanadium ion permeability, good in chemical and thermal stability, suitable for all-vanadium redox flow batteries, simple to operate and low in requirements for equipment and operators.