The invention provides an underground coal mine fracture sealing device and method. The underground coal mine fracture sealing device comprises a water injection pipe, a thick liquid return pipe, a front polyurethane hole sealing section, a back polyurethane hole sealing section, a safety valve, an umbrella type supporting device assembly and a thick liquid injection pipe. The umbrella type supporting device assembly is composed of an umbrella supporting frame, a supporting pipe, a sliding hoop, a piston rod and a piston, and a guide rail groove and the safety valve are arranged on the supporting pipe. When being fed into a drill hole, the umbrella type sealing device shrinks, and then the device is conveniently fed into the drill hole. When high-pressure water of which the initial injection pressure is 5MPa is injected, the umbrella type supporting device is opened, the piston is pushed forwards, the umbrella type supporting frame is unfolded and penetrates into coal, the high-pressure water continuously squeezes the hole sealing sections, and then the umbrella type supporting frame penetrates into the coal deeper and deeper. When the water injection pressure is increased to be larger than or equal to 15MPa, pressure water breaks through the safety valve to carry out fracturing operation on the drill hole, and therefore pressure loading capacity of the hole sealing sections is greatly improved, water injection pressure is increased, and the fracturing effect is improved. The underground coal mine fracture sealing device is simple in structure, easy to operate, high in pressure loading capacity, and good in sealing effect.