A controlled-release composition and controlled-release microspheres containing an exendin as an active ingredient, and a method of preparing the same are provided. More specifically, a controlled-release composition containing an exendin as an active ingredient, a biodegradable polymer with a specific viscosity, and coating materials, having high bioavailability and showing sustained release of the active ingredient in an effective concentration for a certain period without an excessive initial burst of the active ingredient; controlled-release microspheres containing a core including an exendin as an active ingredient and a biodegradable polymer, and a coating layer coating the core; and a method of preparing controlled-release microspheres including the steps of mixing an exendin, a biodegradable polymer, and a solvent, removing the solvent from the mixture to prepare hardened microspheres, and coating the hardened microspheres to form a coating layer on the surface of each microsphere, are provided.