The　stability　of　the　middle　soil　pillar　(MSP)　is　considered　to　be　a　crucial　factor　in　ensuring　the　safety　during　the　construction　of　asymmetric　parallel　tunnels.　In　this　paper,　both　geomechanical　model　tests　and　numerical　analysis　were　carried　out　to　investigate　the　mechanical　characteristics　of　the　MSP　between　asymmetrically　closely-spaced　parallel-tunnels.　First,　large-scale　three-dimensional　(3-D)　geomechanical　model　tests　were　conducted　to　study　the　stress　release　laws　and　mechanical　behaviors　of　the　MSP　during　the　excavation　and　overloading　conditions.　Then,　a　numerical　model　was　implemented　based　on　the　physical　model　for　verifying　the　MSP＇s　stress　response　obtained　from　the　model　tests.　Failure　degree　index　(FDI)　was　put　forward　to　evaluate　the　degree　of　disturbance　to　the　MSP　and　to　analyze　its　damage　extent　during　tunnels　excavation.　Furthermore,　the　effect　of　construction　parameters　(i.e.,　support　strength　and　excavation　sequence)　on　the　damage　behaviors　of　the　MSP　were　discussed　by　extended　numerical　analysis.　The　model　tests　results　show　that　the　stress　was　mainly　released　from　the　upper　and　lower　ends　of　the　middle　pillar　to　lateral　direction　in　sand　ground.　Both　the　horizontal　stress　release　and　horizontal　stress　release　rate　of　the　MSP　increased　along　with　the　external　applied　loads.　Meanwhile,　the　overloading　process　only　increased　the　stress　of　surrounding　soil,　while　the　load-bearing　pattern　of　the　MSP　was　basically　unchanged.　In　addition,　the　numerical　results　indicate　that　due　to　the　differences　in　the　cross-section　dimensions　and　construction　parameters　(e.g.,　support　strength　and　excavation　sequence)　of　the　closely-spaced　parallel-tunnels,　the　final　damage　pattern　of　the　MSP　was　asymmetrical.　©　2020