The invention discloses a synthesis method of biodegradable copolyester, and belongs to the technical field of high molecular material synthesis. The method comprises the following steps of (1) mixing raw materials including a dicarboxylic acid mixture and 1,4-butanediol or a dihydric alcohol mixture and succinic acid according to a certain alcohol-acid mol ratio, adding a mixture to a polymerization kettle, adding a catalyst, and reacting at 150-180 DEG C under inert-gas protection; (2) carrying out pre-polycondensation reaction under the conditions of pressure being 300-1500Pa and temperature being 170-200 DEG C; (3) finally raising the temperature to 200-230 DEG C and reducing pressure to 10-100Pa, and carrying out polycondensation for 1-5 hours to obtain products, namely random linear copolyesters including polybutylene succinate-co-terephthalate (PBST) and polybutylene-co-ethylene succinate (PBES). By adopting a composite catalysis system, the synthesis method has the advantages that an antioxidant and a chain extender are not needed to be added, a product has high molecular weight which can achieve 80000-150000, and the reaction temperature is lowered; the synthesis method is suitable for industrial production.