This　paper　aims　at　investigating　the　instability　mechanism　that　occurs　during　the　under-crossing　of　an　existing　pipeline　using　model　tests　and　numerical　simulations.　The　strain　in　the　pipeline,　earth　pressure,　surface　settlement,　and　mechanisms　of　stratum　instability　during　the　process　of　instability　have　been　obtained.　The　research　findings　reveal　three　distinct　growth　stages　in　the　longitudinal　deformation　of　the　existing　pipeline,　with　the　strain　growth　rate　during　the　rapid　growth　phase　being　2–3　times　higher　than　that　observed　during　the　slow　growth　phase.　The　pipeline　displays　a　deformation　pattern　with　tensile　stress　in　the　upper　section　and　compressive　stress　in　the　lower　section,　due　to　the　uneven　force　distribution　along　its　length.　In　the　absence　of　pipelines,　the　distribution　of　surface　collapse　exhibits　a　distinct　“V”　shaped.　As　the　clear　distance　between　the　existing　pipeline　and　the　tunnel　(CDPT)　increases,　there　is　a　transition　from　a　“partial　W”　to　a　“positive　W”　type　distribution.　The　presence　of　the　pipeline　exerts　the　most　significant　inhibiting　effect　on　the　soil　directly　above　it.　In　cases　where　there　is　no　pipeline　or　the　CDPT　is　small,　soil　leakage　primarily　occurs　in　front　of　the　excavation.　As　the　CDPT　increases,　soil　leakage　occurs　around　the　pipeline　and　the　instability　range　of　the　stratum　significantly　expands.　The　stable　strata　above　the　pipeline　gradually　widen,　and　the　area　with　a　significant　influence　range　shifts　from　the　right　side　to　the　left　side　of　the　existing　pipeline.　©　2024　Elsevier　Ltd