Abstract: Disclosed are a method for preparing a carboxyl cyclic acid anhydride and a prepared product thereof. The disclosed method includes the steps of reacting a compound of formula (I) with a cyclization reagent to produce the compound of formula (II) and an acid, and using an epoxy compound as an acid scavenger. The disclosed method is low in cost, and has low requirements for reagents, places and equipment required for experiments, mild conditions, a wide application range of substrates and a high tolerance of functional groups. The synthesis route can recycle some solvents and other high value-added by-products, with less waste liquid discharge and easy amplification. It also has extremely high industrial and academic value, and can greatly promote the rapid mass production and application of products such as polyamino acid, polyhydroxy acid (polyester), and polymercaptic acid (polysulfide).