The invention discloses a dosing device for a closestool water tank, which comprises a dosing sliding pole with a floater fixed at the lower part and an inclined surface as the top, and a shell which is arranged above a highest water level of the closestool water tank. The shell comprises a guide cavity which is provided with a drug hole inside and a dosing hole on an upper end and guides the dosing sliding pole, and a drug storage cavity in which a soluble spherical disinfecting agent is stored. A highest point of the top inclined surface of the dosing sliding pole is lower than a lowest point of the drug hole after water in the closestool water tank is completely drained at the latest; and a lowest point of the top inclined surface of the dosing sliding pole is higher than a highest point of a lower opening part of the dosing hole after the water surface in the closestool water tank rises to a highest level at the latest. The dosing device fully realizes the automatic dosing of the (spherical) solid disinfecting agent, fully guarantees the appropriate concentration of effective components of the disinfecting agent in the water of the closestool water tank, avoids the waste caused by overlarge concentration; at the same time, the dosing device also avoids the problem that the disinfection effect is gradually weakened along with the time.