With the construction and development of some large important projects, such as high-rise buildings, subways, earth dams and so on, deep foundation pit has been a critical question in geotechnical engineering.Research on m echanical mechanism of soil in the condition of excavation unloading is the key to solve the problems in deep foundation pit engineering.A series of experimental researches on the deformation character were made of twenty undisturbed saturated samples of silt clay from a deep foundation pit in Zhuhai, gongbei tunnel project of Hong kong-zhuhai-macao bridge at KO consolidation and drained shear test.Based on damage soil mechanics, an elastoplastic model considering unloading damage for soil is set up, in which damage potential function is built with unloading stress path test results.This model of soil is introduced to finite element method analysis for excavation engineering and the FEM equation is proposed.A corresponding FEM program is compiled.The deep excavation engineering in Zhuhai, gongbei tunnel project of Hong kongzhuhai-macao bridge is analyzed using this FEM program.The calculated results are studied from the aspects of excavation deformation, earth pressure on retaining wall, evolution ot damage variables, etc.It has shown that the elastoplastic unloading damage model of soil is feasible and applicable to deep excavation engineering.