The invention provides a curing agent for carboxyl resin as well as a preparation method and an application of the curing agent. The curing agent is prepared from main raw materials including phenyl polyacid, phenyl polyphenol, sodium hydroxide, allyl bromide, epichlorohydrin and a catalyst through joint reaction. The curing agent contains both epoxy groups and double-bond groups, so that the curing agent can be subjected to a reaction with carboxyl groups in branch chains of carboxyl acrylic resin and can also be subjected to a reaction with double bonds remaining in main chains, and a densefilm with higher crosslinking density is formed.