Cave 3，as the largest excavation in the yungang grottoes, came to an abrupt end during its excavation process. The special structure and natural weathering for thousands of years have resulted in a variety of diseases, among which the collapse of back room roof is the most serious. By means of rock testing，physical exploration and borehole exploration, the failure forms of roof were found out. Under the premise of temporary support, the roof is strengthened by measures such as crack grouting and anchor rod pulling. The analysis of crack, tilt and stress monitoring data of 120 days after completion shows that the reinforcement effect is good and it has a wide reference significance.