The invention discloses a coal seam hard top plate fracturing weakening method. The coal seam hard top plate fracturing weakening method comprises the following steps that firstly a fracturing hole is drilled in the top plate, then a thermal insulation casing pipe is utilized to inject liquid nitrogen into a top plate rock stratum in a pulse mode, the coupling effect of pulse fatigue damage and liquid nitrogen cold shocks can weaken the strength of the top plate rock stratum itself to a large margin, and when gasification pressure exceeds the rock stratum tensile strength, fractures are formed around the fracturing holes, and finally nitrogen is recycled. The method integrates the advantages of fatigue weakening, the cold shocks and gasification inflation, on one hand, by relying on a gas-liquid two-phase flow pressure group of which the size is gradually increased formed in a limited drilling hole space, the temperature inside the rock stratum is sharply reduced, and the generation of the thermal stress is induced, on the other hand, a pulse type injection method makes nitrogen gas-liquid two-phase flow quickly flows to the fractures, the fracture expanding velocity is increased, and finally effective collapse of the top layer rock stratum is achieved. The method is simple and convenient to construct, low in cost and wide in application range.