Abstract: Construction in expansion soil layer is often adversely affected by swell-shrink, fissure, over-consolidation characteristics of expansion soil, the mechanics and deformation characteristics of expansion soil deep foundation pit during excavation is studied based on coupled thermo numerical simulation. At first, the expansion force of undisturbed expansion soil is measured by laboratory test, and then a ring knife soil sample is simulated using coupled thermo module of FLAC3D,to derive expansion coefficient under target expansion force, finally mechanics and deformation characteristics of deep foundation pit in excavation process under Above-mentioned expansion coefficient is calculated, taking a Yangzhou expansion soil foundation pit as an example, and comparing with the measured data of the foundation pit. The main research conclusions are as follows:(1) Contour line of retaining structure lateral deformation is shaped like a fish belly, the maximum lateral deformation occurs in a certain range above the excavation face, the maximum lateral deformation is approximately(0.15%~0.30%) H; (2) The curve of vertical column displacement is gentle, the value is approximately(-0.04%~0.14%) H; (3) The surface settlement trough behind the retaining wall is type, the influence range is beyond 2.5He,the maximum surface settlement is vm=(0.05%~0.15%) H, the relationship between maximum surface settlement and maximum lateral deformation of retaining structure is approximately vm=(0.33~0.56)hm; (4) The earth pressure values are within the range of the earth pressure envelope by measurement, the value is substantially 1.07He.