Abstract: During the past thousand years, Yun-gang Grotto has suffered a lot from the seepage hazard, and at the same time, the establishing column rock's stability of the grotto has reduced because of the dissected by the tectonic joint and unload crack. The weathering effect on the establishing column rock in front of grotto 9, grotto 10 and grotto 12 is the most serious. The stability of the columns has great effect on the safety of grotto, so relative reinforcing measures should be taken as soon as possible. The author has carried out site investigation and hazard research, some rock and water samples had been taken, and relative testing had also been carried out. Three-dimensional numerical simulation has been used to find out the load forced on the column. And the stress distributions, strength and safety factors of the column rock have also been researched. All these work gave scientific evidence for the grotto's prevention and reinforcement designing. Research result shows that, the pressure-stress on the top of the column rock of grotto 9, grotto 10 and grotto 12 have been changed from 0.45MPa, 0.44MPa (before the grotto is shaped) to 1.29~1.31MPa, 0.94~0.96MPa (after the grotto is shaped), which has increased to 2~3 times of the original stress. The biggest load was on the top of column 4 in grotto 10, which is 3741kN. The highest uniaxial compressive strength of the column rock is 1.95MPa. According to the criterion, all the six columns are in state of danger, and the grotto 9, grotto 10, grotto 12 should all be reinforced.