Currently　the　less　effort　of　research　on　the　theory　of　space　earth　pressure　behind　retaining　wall　adjacent　to　existing　basements　exterior　wall　has　been　made.　the　existing　plane　strain　state　theories　can＇t　satisfy　the　calculation　requirement　of　the　earth　pressure　behind　retaining　wall　adjacent　to　existing　basements　exterior　wall　when　the　length　height　ratio　B/H　of　retaining　wall　is　smaller.　In　this　paper,　soil　arching　effects　principle　is　introduced　into　the　space　earth　pressure　calculation　model　built　by　GU[8]　and　a　model　for　calculating　space　earth　pressure　is　developed　considering　soil　arching　effect　and　existing　basements　exterior　wall.　Based　on　the　relationship　between　the　failure　space　and　the　distance　of　two　walls,　the　soil　between　the　retaining　wall　and　the　existing　basement　exterior　wall　is　divided　into　four　districts,　I,　II,　III,　IV.　Formulas　for　calculating　the　space　active　earth　pressure　in　every　district　behind　the　retaining　wall　are　derived　by　establishing　horizontal　and　vertical　static　equilibrium　differential　equation　of　the　horizontal　differential　element　taken　from　every　district.　The　Formulas　can　be　applied　to　calculate　the　active　earth　pressure　wherever　behind　the　retaining　wall.　Furthermore,　the　method　of　resultant　force　of　the　space　active　earth　pressure　and　its　position　are　also　put　forward.　The　spatial　distribution　of　active　earth　pressure　behind　the　retaining　wall　can　be　seen　visually　through　the　calculation　of　the　numerical　examples.　Two　conclusions　can　be　drawn　as　follows:　When　the　spatial　effect　exists,　the　active　earth　pressure　distribution　considering　the　soil　arching　effect　along　the　retaining　wall　is　quite　different　from　that　when　the　retaining　wall　is　in　the　plane　strain　state;　that　is,　the　active　earth　pressures　behind　both　ends　of　the　retaining　wall　are　much　smaller　than　that　in　the　plane　strain　state.　The　position　of　the　active　earth　pressure　resultant　force　point　considering　spatial　effect　is　higher　than　that　in　plane　strain　state;　and　when　the　B/H　gets　smaller,the　influence　of　spatial　effect　on　the　distribution　of　active　earth　pressure　along　the　retaining　wall　and　the　position　of　the　resultant　force　point　will　increase.　©　2016,　Science　Press.　All　right　reserved.