The invention relates to a water tank of a toilet bowl. A tank cover is arranged on an opening formed in the upper end of a tank body, a water releasing button is fixed to the tank cover, the lower end of the water releasing button abuts against a drain valve, the drain valve is fixed to a drain pipe installation hole formed in the bottom wall of the tank body in a sealing mode, a first four-way joint is fixed to a drain valve water outlet formed outside the tank body in a sealing mode, a second four-way joint is communicated with the lower end of an overflow pipe in a sealing mode through a first hose, and a third four-way joint is communicated with a toilet bowl water flushing port in a sealing mode through a drain pipe; the overflow pipe is fixed to an overflow pipe installation hole formed in the bottom wall of the tank body in a sealing mode; the drain valve is communicated with a water outlet of a low-level water closing valve and a water outlet of a high-level water closing valve in a sealing mode separately; a domestic wastewater inlet connector is fixed to the upper end of the side wall of the tank body. The water tank is simple in structure and can fully utilize domestic wastewater to achieve the water saving purpose.