A disposal device comprises a raw material conveyor, a raw material mixer, a liquid waste conveying pipeline, an additive tank, a powder waste conveyor, an output pump, a liquid supply pump, a liquid supply manifold, an output manifold, a mixed liquid conveying pipeline, a high-pressure injection pump, a high-pressure pipeline, and a wellhead sealing device. A method of employing the device includes: drilling a well; forming a fracture in the granite stratum; preparing a raw material; and injecting, by using the disposal device, a sand-carrying feed liquid from a high-pressure injection pump into the fracture of the underground granite stratum, so as to perform solidification.