The utility model discloses a fire-fighting automatic pressure relieving device which mainly comprises a fixed frame, a cover plate and a pressure adjusting device, wherein the cover plate is connected with the upper part of the fixed frame by a hinge; and the pressure adjusting device is arranged at the connecting position of the bottom part of the fixed frame and the cover plate. Due to the fire-fighting automatic pressure relieving device, the opening and the closing are realized by rotation of a U-type bearing plate of the cover plate around a hinge fixed on the fixed frame. The fire-fighting automatic pressure relieving device is provided with the pressure adjusting device which consists of an adjustable ball latch seat and an adjustable ball latch plug. The adjustable ball latch seat is arranged on the fixed frame, and the adjustable ball latch plug is arranged on the cover plate. By adjusting a set screw on the adjustable ball latch seat, a spring is compressed and the control of clamping force to the adjustable ball latch plug is realized, thus reaching the set opening pressure value of a protective area.