The invention discloses a self-pressurization protecting device and method thereof for keeping an expansive soil cut slope stable. The device comprises an anchor rod, a first circular plate, a second circular plate and a high-strength spring, the anchor rod is sleeved with the first circular plate and the second circular rod, the high-strength spring is arranged between the first circular plate and the second circular plate, a nut is arranged on the first circular plate, and the anchor rod is fixedly sleeved with the nut. By means of the self-pressurization device, the problem that the expansive soil cut slope tends to be unstable under the action of atmosphere is solved, the device is applicable to various types of expansive soil, meanwhile, the arrangement form is simple, the construction operation is simple and convenient, the device can be used for provisional protection as well, and the construction application prospect of the device is broad. The action mechanism of the protecting device conforms to the working rules of the expansive soil cut slope, and engineering characteristics of the expansive soil are taken into full consideration in the protection design; the construction efficiency is high, the arrangement is flexible, and the device is applicable to various soil layers; the device can be made of metal materials or bamboo or wood according to the protection time length, and the effect to the environment is little.