The invention discloses a heat stabilizer for PVC (polyvinyl chloride) and a preparation method and application thereof. The preparation method includes: subjecting tung maleic anhydride and para aminobenzoic acid to anhydride aminolysis reaction to obtain N-(3-amino-benzoyloxy) tung maleic amic acid; performing one-step reaction of the prepared N-(3-amino-benzoyloxy) tung maleic amic acid and metal oxides or metal hydroxides in solvent to prepare liquid N-(3-amino-benzoyloxy) tung maleic amic acid metal salt; compounding the prepared N-(3-amino-benzoyloxy) tung maleic amic acid calcium salt with zinc salt to obtain a liquid composite heat stabilizer. The prepared calcium-zinc liquid composite heat stabilizer has advantages of nontoxicity, freeness of heavy metals, environment friendliness and the like and endows PVC products with high heat stability and transparency. The preparation method is simple in process route and mild in condition, raw materials which are selected from natural environment-friendly renewable resources are rich in source and low in cost.