Abstract: The Longyou grottoes(Xiaonanhai Stone Room), as key protected cultural relics in China, hold significant value in engineering science. Over the past two decades, roof cracks within the caverns have developed rapidly, posing a substantial threat to their stability. Therefore, comprehensive research on the mechanism of roof cracking and long-term strength in the Longyou Grottoes Reserve is imperative. Niuchang No. 1 ancient cavern, located adjacent to the Longyou Grottoes, offers relatively simple geometric conditions that serve as an exemplary case for analyzing both roof cracking mechanisms and long-term strength inversion. Based on field investigation, the roof crack distribution of Niuchang No. 1 cavern was initially obtained. Subsequently, using three-dimensional laser scanning data, a high-precision three-dimensional numerical model of the cavern was established and its stress field was simulated. The results of theoretical analysis and numerical simulation showed that the tensile stress concentration area of the roof of Niuchang No. 1 cavern is in good agreement with the distribution of the main cracks, with stress concentration acting as a driving factor for both initiation and expansion of these cracks. The layering of roof rock mass increases the tensile stress concentration of the immediate roof, which not only reduces the bending strength of the roof but also affects the number and distribution characteristics of cracks. The tensile stress zone above 0.35 MPa can cause the initiation of roof cracking, and this value can be used as the long-term tensile strength reference value of surrounding rock at the centennial scale. The research findings can offer valuable insights into the mechanism of long-term stability and tourism safety planning of caverns in the Longyou Grottoes Reserve.