A mechanical sealing and flushing method and device of a seafloor mud lifting pump comprises a seawater pump, a pump outlet, a pump inlet, an impeller, an O-type ring and a seawater flushing pipe. The seawater pump is arranged in seawater and is coaxial with a motor, the pump outlet is formed in the seawater pump, the pump inlet is formed in a front end cover of the seawater pump, the impeller is arranged inside the seawater pump and is connected with a motor shaft through keys, the O-type ring is arranged between the seawater pump and the front end cover of the motor, and the seawater flushing pipe is connected with the pump outlet and a sealing and flushing opening. The seawater pump is immersed in the seawater, the pump inlet is directly communicated with the seawater, the seawater is boosted through the effect of the impeller inside the seawater pump, and seawater flowing and flushing to a sealing face are achieved. The mechanical sealing and flushing method and device of the seafloor mud lifting pump is simple in structure, sealing liquid does not need to be drawn from a sea surface to flush a circulation pipeline, the problem of sealing face flushing of the seafloor mud lifting pump of a seafloor mud lifting drilling well can be effectively solved, meanwhile, the mechanical sealing and flushing method and device of the seafloor mud lifting pump can be applied to ocean mining and other seafloor solid-liquid two-phase conveying pumps, and the demands for mechanical sealing and flushing of deep sea seafloor solid-liquid two-phase conveying pumps are met.