The invention discloses a method for reducing silt coal slime in an underground water sump of a large water coal mine. The method comprises the steps as follows: firstly, before working face mining, a water draining roadway is arranged in a position lower than the elevation of a transport belt roadway, through holes are formed in the transport belt roadway at intervals, and the transport belt roadway is communicated with the water draining roadway through the through holes; a vibrating sieve is mounted at the tail end of the water draining roadway and a coal transporting roadway is arranged behind the vibrating sieve; transport equipment is arranged in the coal transporting roadway; two water storage deposition pools are arranged on one side of the water draining roadway and water inlet gates are arranged at the inlets of the water storage deposition pools; the water storage deposition pools are communicated with the underground water sump; when mining, a water coal body is discharged into the water draining roadway from the through holes and filtered by the vibrating sieve, and filtered coal is transported to an underground coal bunker; and coal slime water after filtering is deposited by the water storage deposition pools so as to achieve coal-water separation. According to the method, the coal slime water is purified in advance before entering into the underground water sump, so that the silt amount of the underground water sump is reduced and the water-holding capacity of the water sump is guaranteed.