The present provides a method for preparing a composite material, wherein the method comprises the following steps: (1) compounding a dispersion solution of an ion exchange resin containing a free radical initiator and a high-valence metal compound, with a functional monomer-grafted porous fluoropolymer membrane with a microporous structure by performing solution pouring, tape casting, screen printing process, spraying, or impregnating process; (2) subjecting a wet membrane to heat treatment at 30˜300° C., so that the free radical initiator can initiate crosslinking reaction between the porous membrane and the resin to obtain the composite material.