Abstract: Expansive soil is a weakly permeable stratum with low permeability. It is generally regarded as a relative aquiclude or water resisting layer. Its anti-floating water level is needed to comprehensively consider the site seepage characteristics, fertilizer tank filler characteristics and water-structure interaction, which makes the anti-floating problem very prominent and difficult to deal with. Based on a building failure treatment case in the expansive soil area of Chengdu, through the investigation of the water leakage source of the basement, the monitoring of groundwater flow and the evaluation of the discharge capacity of the drainage system, it is considered that the basement floating is caused by the improper construction control of the fertilizer tank, which makes the high-pressure water penetrate into the bottom of the water resistant plate, resulting in the imbalance of hydraulic conditions. Combined with the FLAC^3D finite difference method, the mechanical behavior characteristics of underground structures under the action of groundwater buoyancy and expansive force are discussed in detail. Combined with the engineering practice, an anti-floating method based on "pressure relief" is proposed. The idea is to open a pressure relief hole in the bottom plate to make the groundwater flow and convert the hydrostatic pressure into kinetic energy without changing the groundwater level, and supplement observation and automatic control measures is carried out to achieve the purpose of anti-floating. The monitoring results of the project show that the flow and head are controllable and the operation cost is low.