A dual-drainage switching mechanism of water tank has: a water container with bottom opening of the inner cavity, an air vent communicating with the inner cavity is arranged on the sealed top; a valve unit used for controlling opens or closes of the air vent; a drain valve controller comprising a partial flush button and a full flush button, and the air vent is opened through the linking and coupling of the full flush button and the valve unit, and the air vent is closed through the linking and coupling of the partial flush button and the valve unit. The dual-draining switch is achieved through hermetically storing or non-hermetically controlling part of water body in the inner cavity of the water container, traditional one-pressing drain valve can be transformed into the draining control mechanism with dual-draining function.