A method for preparing polymeric microsphere by an aqueous two phase emulsion process. A first polymer aqueous solution is provided and the first polymer includes a functional group capable of forming cross-linking. A second polymer aqueous solution is provided, which is acidic and miscible with the first polymer aqueous solution. The first and second polymer aqueous solutions are mixed and stirred to form an emulsion, such that the first polymer solution forms a dispersed phase in a continuous phase of the second polymer solution. The dispersed phase includes a plurality of the first polymeric microsphere, and a solidification film formed by cross-linking of the functional group constitutes a microsphere surface. Finally, the first polymeric microsphere are separated out.