Abstract: The buoyancy of the underground structure is generally calculated on the basis of Archimedes law. However, the influence of surrounding backfilling soil was not taken into account in previous work. This article takes an underground granary that has been constructed as a research object. In this study, the 2:8 lime soil backfilling condition was considered in the indoor scale model tests. The diameters of the underground granary model cylinders were taken as 400 (Model cylinder A), 500 (Model cylinder B) and 600 mm (Model cylinder C), respectively. The whole scale model test consisted of two parts, the pure water test and the 2:8 lime soil backfilling tests. The results of the pure water test indicated that the actual buoyancy calculated based on Archimedes law and the theoretical buoyancy under the self-weight of the model cylinder (including the constrain force of the upper part of the model cylinder, which is also converted into its own weight) were basically consistent. The errors between the actual buoyancy and the theoretical buoyancy for the three models cylinder were 0.03%, 1%, 3%, respectively. In the 2:8 lime soil backfilling test, the water was injected slowly, the displacement was monitored by the displacement meter and the reaction force at the upper surface was measured by the pressure sensor. The numerical variation of displacement and constraint force during the whole process of the model cylinder floating was analyzed. The displacement mutation value lags behind the constraint force mutation value. When the pressure sensor reading changed significantly but the displacement meter reading did not, the actual water level served as the warning water level of the model cylinder. When the pressure sensor and the displacement meter reading changed significantly together, the actual water level was used as the actual floats water level of the model cylinder. Taking the warning water level and the actual floating water level as the theoretical basis for calculation, obtained the theoretical resistance to buoyancy under the self-weight of the model cylinder and the actual resistance to buoyancy. When reaching the warning water level, the actual resistance to buoyancy of model cylinder A, B, C were 1.93, 2.43, 1.66 times respectively as the theoretical resistance to buoyancy. When reaching the actual floating water level , the actual resistance to buoyancy of model cylinder A, B, C were 2.15, 3.36, 2.96 times respectively as the theoretical resistance to buoyancy. When backfilling 2:8 lime soil around an underground granary, calculating the buoyancy using Archimedes' law is safe. Considering the safety reserve comprehensively, the 2:8 lime-soil backfilling condition of underground granary, when reaching the warning water level, the actual resistance to buoyancy is 1.5 times of the theoretical resistance to buoyancy,and when reaching the actual floating water level, the actual resistance to buoyancy is 2 times of the theoretical resistance to buoyancy. The research results are able to provide a reference for the anti-buoyancy design of underground granaries and other underground structures in the future.