<p>Based on the magnitude and geometric distribution of the Failure Approach Index (FAI) of the rock mass surrounding the mining stope, including the maximum values of the roof and the wall for magnitude indicators, and the distribution shape, the extent of distribution, the local limit depth for geometric distribution indicators, a multi-index subjective and objective fuzzy mathematical method is adopted to construct a stope stability evaluation method for horizontal and gently inclined thin vein mining. Similar stopes with varying length-to-height ratios (L/H) are mined through laboratory similar tests and numerical simulations. The mechanical responses of the surrounding rock mass are monitored to obtain the Failure Approach Index (FAI). Meanwhile, the stability of the stope is assessed using the novel such stability evaluation method. Numerical simulations and laboratory tests show basically consistent results. Stopes with L/H of 0.5 and 1 are considered excellent. Those with L/H of 2, 3, and 4 are good. Specifically, laboratory test rates the stope with L/H of 5 as moderate, and numerical test rates it as good. Stope with L/H of 6 is moderate, and stope with L/H of 7 is poor. Stopes with L/H of 8, 9, and 10 are rated as very poor for stability. Finally, the evaluations from both numerical simulations and laboratory tests are fundamentally consistent with the final patterns of fractures and damage. This consistency, as verified by on-site observations, proves that the stope stability evaluation method is reliable and applicable.</p>