A mining-coal bed nearly horizontal gas extraction borehole sealing method is mainly suitable for gas extraction boreholes at sites like coal faces in underground coal mines and coal roadways. The sealing method mainly includes two parts: static sealing and dynamic sealing. For a gas extraction borehole, static sealing is performed firstly and includes two times of grouting: for the first grouting, expansion cement grout is adopted to seal positions outside a fissure zone, and for the second grouting, slurry is adopted to seal positions inside the fissure zone; after a period of time, dynamic sealing is performed and includes that the expansion cement grout and the slurry are constantly grouted into the borehole by utilizing underground compressed air to enable the borehole and surrounding fractures to be always filled with the expansion cement grout and the slurry so as to realize dynamic sealing. By the method, grouting fluid can be enabled to effectively seal space inside the borehole and coal fracture areas surrounding the borehole in real time, and maximization of space and fracture sealing is realized. The mining-coal bed nearly horizontal gas extraction borehole sealing method is simple in structure, convenient to operate, low in cost and evident in effect.