The invention discloses a cyclic trimetric polybutylene terephthalate preparation method. The method comprises the steps: (a) preparing bis(4-(4-allyl terephthaloyl oxy)butyl)polyethylene terephthalate (III) from bis-(4-hydroxyl butyl)-polyethylene terephthalate (I) and 4-(allyloxy carbonyl)benzoic acid (II) under action of a condensing agent and an auxiliary reagent; (b) preparing cyclic trimetric polybutylene terephthalate precursor (IV) by ring closure of the bis(4-(4-allyl terephthaloyl oxy)butyl)polyethylene terephthalate (III) under the action of a Grubbs catalyst and lewis acid; (c) hydrogenating the cyclic trimetric polybutylene terephthalate precursor under the action of Pd/C to obtain cyclic trimetric polybutylene terephthalate (V). The preparation method disclosed by the invention has the advantages that the raw materials are cheap and easy to obtain, a synthesizing route is simple and short, a yield is high, and conditions are easy to control.(The formula is shown in the description.).