Abstract: The big irregular mined-out area was formed after years of underground mining in a gold-silver mine. The exposed area is approximately 0.8 million square meters with the volume of 95, 000 cubic meters. To remove the hidden danger of large -scaled ground pressure, the instability district must be determined for processing ground pressure. The numerical simulation results show that the 94 m phase roof exist big tensile stress and the submersion displacement quantity is biggest, this phase roof is preliminary judged as the instability district. On these bases, the similarity simulation experiment discovered that the 94 m phase roof position first presents the crack, and collapses first. This phase roof is further proved as the instability district. It provides scientific basis for processing ground pressure.