The invention provides a mechanical sealing plug capable of quickly and simply stopping mortar/water and a mounting method of the mechanical sealing plug. The mechanical sealing plug comprises an inner mortar/water injection pipe and a casing pipe encased outside the inner mortar/water injection pipe. A first steel washer, a rubber plug bush and a second steel washer which are sleeved on the inner mortar/water injection pipe are arranged at the head of the inner mortar/water injection pipe. The rubber plug bush is arranged between the first steel washer and the second steel washer. The tail of the inner mortar/water injection pipe is provided with an outer threaded structure. A first steel backing plate and a first nut used for preventing the first steel backing plate from sliding are assembled on the outer threaded structure. The head of the casing pipe is adjacent to the second steel washer. A plurality of wing plates and a second steel backing plate are uniformly arranged on the outer side of the tail of the casing pipe. A jack is arranged between the first steel backing plate and the second steel backing plate. The mechanical sealing plug has the advantages of good sealing performance, simplicity and labor saving in operation, no need of special processing, convenience in mounting and demounting, high recyclability and the like.