The invention relates to a method for dismantling compressed concrete beams in a deep-foundation-pit unsupported enclosure system. The method is characterized by comprising the following steps that (1) according to the section of one compressed concrete beam in the deep-foundation-pit unsupported enclosure system, a counter-force support is manufactured; (2) the counter-force support is fixed to the compressed concrete beam through a horizontal shear resistance bolt; (3) the other pairs of counter-force supports are mounted a certain distance away from the position where the counter-force support is mounted by repeatedly carrying out the step (2); (4) horizontally-arranged jacks with the equal number are arranged between every two counter-force supports; (5) jacking force is applied to thejacks, the jacking force value is matched with the pressure borne by the compressed concrete beam, at the moment, a pressureless beam is formed, cutting is implemented on the pressureless beam, and the cutting face is parallel to the beam section; (6) after cutting, force unloading is conducted on the jacks step by step according to the regulation and time till stress of the jacks is zero; and (7) the other compressed concrete beams in the deep-foundation-pit unsupported enclosure system are dismantled continuously according to the steps. According to the method, 'zero' damage is caused to amajor structure, the directengineering cost is greatly reduced, the construction period is shortened by 2/3, and the benefit in the construction period is huge.