Earth　Pressure　Balanced　(EPB)　tunneling　boring　machine　(TBM)　establishes　its　dynamic　earth　pressure　balance　through　conditioning　the　excavated　soil,　and　discharges　the　extracted　soil　in　screw　conveyor　smoothly.　In　this　topic　we　refers　the　fruits　of　the　slump　tests　on　conditioning　the　excavated　soil　with　foam,　and　conducts　experiments　on　the　mechanism　of　EPB　conditioning　the　excavated　soil　in　microscopic　point　of　perspective.　Assisted　by　Industrial　CT　Observing　System,　microstructure　images　of　the　unconditioned　and　conditioned　soil　are　obtained.　The　images　are　processed　with　computer　program　written　to　get　the　different　parameters　of　micro-structure　between　the　conditioned　soil　and　unconditioned　soil.　The　results　indicate　the　changes　on　micro-structural　feature　affected　by　foam.　The　experiments　supply　references　for　conditioning　the　excavated　soil　with　foam　in　excavation　of　urban　tunnels　with　EPB.　©　2013　American　Society　of　Civil　Engineers.