A controlled release preparation of insulin and its method are provided. The controlled release preparation of insulin contains microparticles obtained by microencapsulation of uniform microcystals of insulin using biodegradable polymeric materials. Since the denaturation of insulin that may occur during microencapsulation is reduced, the stability of the preparation can be increased. Also, the ratio of insulin to a polymer carrier is increased, which is suitable for pulmonary delivery. Further, the controlled release preparation of insulin can continuously exhibit pharmaceutical efficacy in vivo in a stable manner for an extended period of time.