The invention relates to a water closet and especially provides a water-saving device for a water closet. The water-saving device mainly comprises a water draining valve (1), a water reservoir (2), an overflow pipe (3), a water pump (4), a filtering device (7), a water pumping pipe (8) and a washbasin draining pipe (9). The water-saving device is characterized in that the left end of the water draining valve (1) is connected with a bottom right corner of the water reservoir (2); the lower part of the overflow pipe (3) penetrates through the bottom of the water reservoir (2); the water pump (4) is arranged at the bottom center of the water reservoir (2); the filtering device (7) is arranged at the center of the outer surface at the upper end of the water reservoir (2); the lower end of the water pumping pipe (8) is connected with the water pump (4); the upper end of the water pumping pipe (8) is connected with the bottom of a water tank (13); the lower end of the washbasin draining pipe (9) is connected with the upper end of the filtering device (7); the upper end of the washbasin draining pipe (9) is connected with the bottom of a washbasin (12). According to the invention, reusable water in daily life is comprehensively utilized, the water quality is guaranteed and the water-saving effect is excellent.