The changes of three-dimensional stress state of the surrounding soil mass during the excavation of a foundation pit project in Hedong District, Tianjin City was studied. Three-dimensional earth pressure boxes are used to monitor the excavation process of the foundation pit on site, then the monitoring results are compared with the simulation results of the finite element model to study the variation law of three-dimensional stress, principal stress and principal direction of soil. The results show that the effect of foundation pit excavation on the surrounding soil mass is neither pure loading nor pure unloading; the influence of excavation process on the three-dimensional stress state of the surrounding soil mass is mainly reflected by the normal stress, with little influence on the shear stress. The main direction will deflect with the excavation of the foundation pit and the maximum deflection angle can reach 110°. During the excavation of the first section, three deflection angles of α,β,γ have a tendency of approaching 0°. The change curves of α and β generally show the characteristics of symmetry with 0° as the axis.