In　this　paper,　for　a　deep　foundation　pit　of　an　underground　station　in　Changchun,　the　instability　crisis　and　its　mechanism　in　the　ventilation　shaft　section　during　the　excavation　of　the　foundation　pit　are　studied　in　depth.　Through　the　analysis　of　the　horizontal　displacement　of　the　pile　body,　the　horizontal　displacement　of　the　pile　top,　the　support　axial　force　of　the　bracing,　and　the　settlement　difference　of　the　shoulders　on　both　sides　of　the　railway　track,　the　major　causes　of　the　excessive　deformation　of　the　foundation　pit　in　the　ventilation　shaft　section　are　discussed.　Then,　the　construction　process　of　the　foundation　pit　is　simulated　using　ABAQUS　finite　element　software,　and　the　instability　mechanism　of　the　foundation　pit　in　the　ventilation　shaft　section　is　analyzed.　The　results　demonstrate　that　the　excessive　deformation　of　the　foundation　pit　in　the　ventilation　shaft　section　is　caused　by　the　coupling　of　the　excessive　stress　release　of　strata　and　the　spatial　effect　of　the　foundation　pit　deformation.　The　spatial　effect　of　the　foundation　pit　deformation　in　the　ventilation　shaft　section　is　remarkable　due　to　the　existence　of　positive　corner　in　the　ventilation　shaft　and　the　absence　of　diagonal　bracing　during　construction,　and　the　pit　wall　produces　torsional　deformation　pointing　to　the　inside　of　the　ventilation　shaft.　The　excessive　excavation　and　unsuitable　support　installation　lead　to　the　excessive　stress　release　in　strata,　which　causes　the　excessive　deformation　of　retaining　piles　and　deteriorates　the　stability　of　the　foundation　pit.　©　2021,　Science　Press.　All　right　reserved.