This　paper　aims　at　proposing　a　calculation　model　to　deal　with　the　face　stability　of　shield　tunnels　in　unsaturated　soil.　Based　on　the　existing　results　of　centrifugal　test　and　numerical　simulation,　a　new　failure　mechanism　is　proposed,　which　combines　the　logarithmic　spiral　with　upper　loosening　arch　mechanisms.　The　failure　mechanism　considers　the　influence　of　the　soil　arching　effect　and　the　logarithmic　spiral　height　coefficient.　Formulas　for　calculating　the　loosening　earth　pressure　of　unsaturated　soil　are　deduced.　Subsequently,　this　model　of　loosening　earth　pressure　is　introduced　into　an　analytical　model　to　compute　the　limit　support　pressure　on　the　tunnel　face.　Compared　with　other　height　coefficients　and　the　degree　of　soil　arching　effect,　when　the　height　coefficient　is　0.95　and　the　degree　of　soil　arching　effect　is　10%-30%,　the　limit　support　pressure　proposed　in　this　paper　is　in　good　agreement　with　the　centrifugal　test　results.　Finally,　comparisons　are　performed　with　the　proposed　model　and　classical　models　as　well　as　typical　centrifuge　model　tests.　The　comparative　analysis　shows　that　the　proposed　theoretical　model　is　in　good　agreement　with　the　numerical　simulation.　The　outline　of　the　failure　zone　by　the　theoretical　model　in　this　paper　is　in　good　agreement　with　the　centrifuge　results.