Disclosed therein is a method for preparing a polyacrylonitrile-based polymer for preparation of carbon fiber having a melting point controlled by selecting an optimal energy of microwave, and a method for preparing a carbon fiber through melt spinning using the preparation method for polyacrylonitrile-based polymer. The present invention uses microwave to control the properties of the polyacrylonitrile-based polymer in a simplified way and prepare the polymer optimized for preparation of carbon fiber precursor through melt spinning for a short polymerization time, and provides a means for mass production of the polyacrylonitrile-based polymer being suitable for melt spinning at a temperature lower than the stabilization temperature and acquiring properties adequate to preparation of carbon fiber through stabilization. Hence, the present invention is expected to contribute to mass production of high-performance carbon fibers at reduced cost.