Abstract：During the past thousand years，Yungang Grotto has suffered a lot from the seepage hazard，and at the same time，the stability of upright column rock masses of the grotto has reduced because the grotto is dissected by the tectonic joints and unload cracks. The weathering effect on the upright column rock in front of grotto 9，10 and 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，and some rock and water samples had been taken and tested. Three-dimensional numerical simulation has been used to study the load on the column，and the stress distribution，strength and safety factors of the column were determined accordingly. All these work provides basis for prevention of potential hazard and reinforcement design for the grotto. Research result shows that，the compressive stress on the top of the column rock of grotto 9， 10 and 12 have grown from 0.45 MPa，0.44 MPa (before the grotto is shaped) to 1.29～1.31 MPa，0.94～0.96 MPa (after the grotto is shaped)，respectively. The biggest load is at the top of column 4 in grotto 10，which is 3 741 kN. The highest uniaxial compressive strength of the column rock is 1.95 MPa. According to the criterion，all the six columns are in state of danger，and the grotto 9，10，12 should all be reinforced.
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