The invention relates to the field of polyvinyl chloride molding and processing, and particularly relates to a composite heat-stable assistant for VC molding and processing and a preparation method thereof. The stable assistant comprises the following raw materials in parts by weight: 100-120 parts of light rare-earth magnesium hydrotalcite, 20-90 parts of curcumin and 20-40 parts of calcium stearate. The preparation method comprises the following steps: evenly premixing light rare-earth hydrotalcite with calcium stearate; mechanically stirring and mixing at 100-120 DEG C for 3 hours, wherein the rotating speed of an agitator is 400-800r/min; cooling the material to 40-60 DEG C, so as to obtain a material pre-mixture; evenly mixing the curcumin with the material pre-mixture; mechanically stirring and mixing at 40-60 DEG C for 2 hours, wherein the rotating speed of the agitator is 300-500r/min; cooling the materials to room temperature, so as to obtain the composite heat-stable assistant. The light rare-earth hydrotalcite-curcumin composite heat-stable assistant is wide in raw material source, low in cost, and friendly to environment, and has excellent strength and toughness.