The present invention provides an automatic water draining weir without external connection to an air source. The automatic water draining weir comprises a water collection groove, an air shield, a gas discharging device, a water outlet pipe and a control device for controlling the gas discharging device, wherein one end of the water outlet pipe is communicated to the water collection groove, the other end of the water outlet pipe is provided with a U-shaped bending pipe, and a mud blocking mechanism is arranged at the position of the channel formed between the outer wall on the upper portion of the water collection groove and the inner wall of the air shield. According to the present invention, by arranging the U-shaped bending pipe and the mud blocking mechanism and by collecting the air overflowed during the aeration of the reaction pool, the inside-air shield pressurization is achieved, such that the water level between the air shield and the water collection groove descends, the mud-water mixing liquid cannot enter the water collection groove, the operation is simple and reliable, the external connection to the gas source and the complex control system are not required, and the investment on the equipment is reduced.