The invention discloses a purpose of a multi-arm star copolymer used as a diverting agent for fracturing and a preparation method of the multi-arm star copolymer. According to the multi-arm star copolymer used as the diverting agent, any one of three-arm star poly lactic-glycolic acid, four-arm star poly lactic-glycolic acid, five-arm star poly lactic-glycolic acid and six-arm poly lactic-glycolic acid is used or several materials from three-arm star poly lactic-glycolic acid, four-arm star poly lactic-glycolic acid, five-arm star poly lactic-glycolic acid and six-arm poly lactic-glycolic acid are mixed to be used according to different particle sizes. The preparation method comprises the steps that racemic lactide or glycolide or a mixture of the racemic lactide and the glycolide is used as copolymerization raw materials to be added into a reaction vessel, and the raw materials are heated to a melted state; an initiating agent and a catalyst are added into the reaction vessel under the nitrogen gas protection, and the reaction is taken to obtain a crude product; the crude product is purified and dried to obtain the multi-arm star copolymer. The multi-arm star copolymer used as the diverting agent has the advantages that the sealing and plugging effect of a conventional diverting agent can be achieved; the automatic degradation and plug removal after fracturing or acid cracking can also be realized; the damage to a reservoir is little. The preparation method is simple, convenient and feasible; the reaction condition is mild; the final product can be prepared only through one-step reaction.