The invention relates to a preparation method of a composite heat stabilizer, belonging to the field of PVC material additive preparation. Given the problems that the existing traditional heat stabilizer is toxic and expensive and has relatively low compatibility with PVC, the invention provides a preparation method of a composite heat stabilizer. The preparation method comprises the following steps: calcining and activating magnesium oxide and calcium oxide; with a rare earth element, soaking and dissolving the magnesium oxide and calcium oxide in acid; enabling reactions in an environment with alkali and mixed gas to form composite liquid; pretreating hydrotalcite for surface modification; calcining at high temperature to expand internal structure; and cooling using the composite liquid, wherein active substances such as calcium and magnesium in the composite liquid are attached to the inside; meanwhile, due to the double-bond structure of the hydrotalcite surface, the compatibility is improved; and the prepared composite heat stabilizer is nontoxic and has good compatibility with PVC.