With the rise of underground structures during urban construction, underground garages can help solve traffic congestion, but the neglect of groundwater buoyancy can cause underground structures to float, damage and collapse. Based on field measurement of displacement variation and ANSYS APDL numerical simulation in this paper, the floor of one single-built underground garage was comparatively analyzed which located in Zhenjiang, Jiangsu province, and its stress and displacement was studied under conditions with or without reduction, then the anti-floating ability of floor was calculated. The results show that when a large area of underground floor is used in the underground garage project, the reduction effect of groundwater buoyancy should be considered in the calculation condition, and the reduction coefficient is large in the middle of the floor and decreases around. Moreover, the law also reflects the stress and displacement distribution of the floor. The measured displacement results are in good agreement with the displacement distribution under the condition of groundwater buoyancy reduction.