Under the influence of years of weathering and self weight stress, a large number of cracks have appeared in the cave wall and its roof of Cave 168, Beishan, Dazu Rock Carvings, resulting in the instability of the surrounding rock at the top of the cave and the danger of instability and collapse. In order to effectively reinforce the broken surrounding rock of the roof of the flat roof grottoes, this paper adopts a roof reinforcement method of hanging anchor bolts according to the geological conditions of the site. Firstly, based on on-site investigation, the distribution pattern of cracks in the roof slab was determined, and the main causes of cracking in the cave roof were summarized. Then, the anchoring length of the anchor rod was calculated using the idea of the New Austrian Method considering the most unfavorable stress on the surrounding rock. Finally, numerical simulation was used to analyze the displacement and stress changes of the cave roof under rainfall conditions using the suspension beam reinforcement method. The results showed that: under the conditions of suspension anchor rod reinforcement, The displacement of its top plate in all directions shall not exceed 1mm; The stress at the cave entrance and inside the cave is relatively concentrated, with the maximum tensile stress being less than the tensile strength value of sandstone, and the surrounding rock of the roof is in a stable state; During the suspension anchor rod support process, the anchoring section mainly bears tensile stress, with an average tensile stress of 4.3MPa, indicating a good stress situation. From the curve data before and after on-site reinforcement, it can be seen that the suspension beam reinforcement method has reduced the fluctuation value of cracks. It can be considered that the grotto maintains a stable state under the conditions of suspension anchor rod reinforcement. This suspension anchor rod reinforcement method provides a reference for the stability support of thin and broken roof slabs similar to flat roof grottoes.