The invention discloses a salt water main pipe automatic deflating device and belongs to the field of freezing method construction technological machines. The salt water main pipe automatic deflatingdevice comprises two connecting pipes arranged on a salt water main pipe, a guiding rod, a telescopic rod, a deflating switch and a floating block, wherein the deflating switch and the floating blockare arranged on the guiding rod. The ends, away from the salt water main pipe, of the two connecting pipes are connected with a guiding rod, a deflating pipe penetrating through the guiding pipe is arranged on the guiding pipe, and a cut-off valve is arranged on the deflating pipe. The guiding rod is connected to the cut-off valve, a baffle is arranged on the position, corresponding to the deflating switch, in the guiding pipe, and a gap for allowing salt water to pass through is formed between the lower edge of the baffle and the inner wall of the guiding pipe. According to the salt water main pipe automatic deflating device, through the floating block, by means of the buoyancy effect, the deflating switch is touched, automatic control is achieved, manpower interference is not needed, theoperation difficulty is reduced, the manpower burden is reduced, control is more accurate and more timely, and the time and construction costs are saved.