The invention relates to the technical field of coal chemical industries, and particularly relates to a polymer modified asphalt binder and a preparation method thereof. The method comprises the stepsthat coal tar pitch, polymerizable monomers and a free radical initiator are uniformly mixed to perform a polyreaction at a certain temperature to obtain polymer modified asphalt, and the temperatureis further increased to perform a deep ageing reaction to obtain binder asphalt. According to the binder and the preparation method, modification is conducted through polymers of the coal tar pitch to improve the comprehensive performance of the asphalt, the demands of the binder can be met, so that the binder has the advantages of simple technology, convenient operation, energy saving and efficiency, the purpose is to overcome the severe defects of single method, rigorous technological conditions, high reaction temperature and difficult coking controlling of existing binder asphalt preparation through a coal tar pitch direct method, and a novel technology and method are provided for coal tar pitch modification.