A drug delivery formulation for treating a mental illness or a central nervous system disorder is disclosed. The drug delivery formulation includes a microcapsule containing a biodegradable polymer and a drug and a crosslinked hydrogel and a method for preparing the same. The drug delivery formulation can slowly release a drug at a constant rate without a burst of drug release in the early stage of drug administration to maintain a constant blood drug level over a long period of time and thus enjoys the advantage of preventing excessive initial release of drug, thereby obtaining a desirable sustained release profile by single administration without necessity for multiple administrations divided at regular time intervals.