The invention relates to the field of nuclear auxiliary cooling water of mega-kilowatt class nuclear power plants and provides an underwater flushing device for expanding the capacity of a spent fuelpool and a method for carrying out underwater flushing on an old lattice by utilizing the underwater flushing device. The underwater flushing device comprises a pipe barrel which is arranged along a plumb bob direction, and a hoisting assembly, a power assembly and a flushing assembly which are arranged on the pipe barrel in sequence from top to bottom, wherein the hoisting assembly is used for driving the underwater flushing device to do translational motion; the power assembly comprises a water pump for leading liquid into the pipe barrel; the flushing assembly comprises a plurality of spraying nozzles; the plurality of spraying nozzles are communicated with the pipe barrel; therefore, the hoisting assembly can be manipulated through a crane; before flushing, the flushing assembly is slowly put into a grid unit of the lattice and the water pump is started and adjusted after the flushing assembly is dived to a certain water level; in a flushing process, the hoisting assembly is drivento do up-down reciprocating motion and water flow sprayed by the plurality of spraying nozzles sufficiently acts on an inner wall of the lattice, a bottom nut hole and a bottom groove, so that the washing is more complete and efficient.