In order to study the variation laws of steel support axial force during excavation of a deep foundation pit of Shenyang NorthSouth Expressway Tunnel with burial depth of 17.8 m, the axial force of horizontal steel supports of the supporting system is studied. And then the realtime monitoring of axial force variation during excavation of foundation pit is conducted. Finally, the axial force of steel supports under different conditions is simulated by finite element software MIDAS/GTS, and the axial force nephograms and variation laws of every steel support are obtained. The results show that: (1) With the increase of excavation depth of the foundation pit, the active earth pressure on the foundation pit wall increases gradually, which makes the axial force of every support increase linearly in a short period. (2) The axial force of every steel support in a same crosssection attenuates after the whole erection of the steel supports, whose cause lays on soil stress redistributioncaused balance after rheology behavior of soil behind underground diaphragm wall.