Excavation of Cave 3, the largest-scale cave excavation of Yungang Grottoes, came to an abrupt end during the process. The special structure and natural weathering over thousands of years have resulted in a variety of diseases, among which collapse of the back room roof is the most serious. By means of rock testing, physical exploration and borehole exploration, the causes of roof failure were discovered. The roof has been strengthened by measures such as grouting of cracks and insertion of anchor rods subsequently. The analysis of cracks, tilt and stress monitoring data for 121 days after completion shows that the reinforcement is effective and it has extensive reference significance.