The invention discloses a long-acting rare earth thermal stabilizer for polyvinyl chloride and a preparation method thereof. The long-acting rare earth thermal stabilizer is prepared from aliphatic monocarboxylic acid and polyprotic acid (dicarboxylic acid, tricarboxylic acid, tetracarboxylic acid and/or pentacarboxylic acid) serving as organic components and La3+, Ce3+, Pr3+, Nd3+ rare earth ions, wherein the aliphatic monocarboxylic acid accounts for 10 to 75 percent of the total weight of the carboxylic acid(s). By varying the types and the proportion of the aliphatic monocarboxylic acid and the polybasic carboxylic acid, a series of rare earth thermal stabilizers with high long-acting thermal stabilization performance can be obtained. The rare earth thermal stabilizer of the inventioncan form a good synergistic thermal stabilization effect with zinc soap.