The invention discloses a preparation method of polyethylene glycol diglycidyl ether. According to the method, firstly, through polyethylene glycol 400 and epichlorohydrin, under the action of a catalyst, chloric alcohol ether is generated, and then under the action of alkali, ring-closure reaction is carried out to generate the polyethylene glycol diglycidyl ether. Through a large number of experiments, an optimal synthesis route is obtained, the raw material conversion rate is high, by-products are few, the yield is high (can reach 93.1%), conditions are mild, operation is simple and convenient, and the preparation method of the polyethylene glycol diglycidyl ether is suitable for industrial production.