The invention discloses a wastewater buoyancy flush toilet water pressing device for a rural pit toilet. The device comprises a water tank and a water pressing device, wherein the water pressing device comprises a hollow piston rod which vertically penetrates through the water tank; the lower end of the piston rod is fixedly connected with a piston device connected to the bottom of the water tank in a sealed mode; a water inlet is formed on the upper part of the piston rod; a pedal is arranged on the water inlet; a piston rod side wall hole and a water outlet pipe of which one end is extended into the piston rod and which is connected with a through pipe in the piston device are arranged on the piston rod positioned in the water tank; the other end of the water outlet pipe penetrates through the side wall of the water tank to be connected to a water utilization position; and a plurality of water drainage holes are formed at a corresponding position between the bottom of the water tank and the piston device. According to the device, the water pressing device can be naturally returned to a normal position by using the buoyancy of water without additional structures; and the device uses wastewater, has a simple structure, low cost, long service life and a small number of faults, and is convenient to use and maintain.