Disclosed is a method for preparing sustained release microparticles which release a physiologically active substance for long periods of time. The sustained release microparticles are prepared through a multi-emulsion process. A drug of interest is dissolved or dispersed in each of at least two oils to give at least two primary oil phases or emulsions. Each oil phase or emulsion contains a biodegradable polymer. The least two primary oil phases or emulsions are dispersed in one aqueous phase, synchronously or in succession. From the drug-dispersed solution, the organic solvents are removed to produce microparticles. Therefore, the drugs such as leutenizing hormone releasing hormones can be continuously released in vivo for prolonged periods of time, bringing about an improvement in the therapeutic effect.