Disclosed is a method for preparing a mixed formulation of sustained release microspheres with various compositions by a continuous one-step process. The present method is characterized by preparing the mixed formulation of sustained release microspheres with different compositions by a continuous one-step process by continuously introducing the mixed fluids into a dryer from the two or more different fluids for preparation of sustained release microspheres containing a biodegradable polymer, a drug, an additive and a solvent with different types of contents or both of the components, by controlling the mixing ratios of the fluids according to the time, unlike a conventional method including spray-drying a mixture of microspheres containing a biodegradable polymer, a drug, an additive and a solvent with a single composition. The present method can achieve a desired drug release, which is difficult to be obtained by conventional microspheres with a single composition, by preparing microspheres by a continuous one-step process, and can simplify the conventional complex process of separately preparing several microsphere formulations and then mixing the formulations at a proper ratio to achieve a desired drug release.