Against　the　backdrop　of　explosive　growth　in　freight　demand,　the　negative　impacts　of　the　traditional　road-systembased　port-city　logistics　mode　on　urban　transportation,　environment　and　safety　have　become　prominent　livelihood　issues,　as　well　as　significant　economic　and　social　problems.　Port-city　underground　logistics　system　(PC-ULS)　has　been　widely　recognized　as　an　innovative　way　to　ensure　the　sustainable　development　of　port　cities.　However,　the　influence　mechanism　of　PC-ULS　on　the　port-city　relationships　is　still　unclear.　Thus,　this　paper　attempts　to　explore　the　PC-ULS-based　port-city　relationships　formation　process　and　the　main　influencing　factors　using　a　mixed　qualitative　approach.　The　social,　economic　and　environmental　impacts　of　PC-ULS　on　the　development　of　port　cities　were　systematically　analyzed　using　the　Shanghe　Port　ULS　and　Shanghai　Port　ULS　as　the　case　studies.　The　results　show　that　the　impacts　generated　by　PC-ULS　are　positive　and　widely　distributed　across　the　economic　sphere,　port-city　traffic,　logistics　governance,　urban　equity,　stakeholders,　ports,　and　urban　spaces,　as　well　as　ecological　sustainability.　Furthermore,　the　urban　planning,　technology　compatibility　and　engineering　risk　related　factors　have　a　strong　impact　on　the　PC-ULS　implementation.　A　set　of　incentive　policies　has　been　designed　to　promote　the　effectiveness　of　PC-ULS.　The　findings　provide　a　unique　perspective　on　underground　space　development　and　utilization　to　address　port-city　conflicts,　as　well　as　a　reasonable　implementation　pathway　for　PC-ULS　application.