The present invention relates to a method for preparing a polymeric microsphere and the polymeric microsphere prepared by the method, and specifically, relates to a method for preparing a polymeric microsphere in which a single oil-in-water type of emulsion is prepared using specific polymeric compound and organic solvent and then ammonia solution is added to perform ammonolysis and the polymeric microsphere prepared thereby, a composition for bioactive substance delivery comprising the microsphere and a method for preparing thereof.The preparation method of the present invention does not use a halogen organic solvent which is harmful to human body, and does not use a porogen and uses ammonolysis, and in case of preparing a polymeric microsphere according to the preparation method, a microsphere which the preparation process is simple of and is not harmful to human body and has spontaneous pore-closing functionality and extremely strong porosity and pores.
