As　an　important　part　of　high-pressure　fuel　systems,　the　control　valve　adjusts　the　pressure　by　regulating　the　fuel　delivery　quantity　and　the　withdrawal　of　fuel.　This　study　presents　both　experimental　and　numerical　investigations　into　the　flow　characteristics　within　the　control　valve　of　a　diesel　unit　pump　fuel　system　(UPS).　To　observe　the　fuel　flow　within　the　control　valve,　an　optical　test　rig　based　on　a　baseline　UPS　is　built.　Further,　numerical　simulations　based　on　Computational　Fluid　Dynamics　(CFD)　method　are　conducted　based　on　the　homogenous　two-phase　unsteady　flow　model　using　dynamic　mesh.　The　results　show　that　during　the　valve　opening　period　cavitation　occurs,　directly　affecting　the　fuel-offloading　process　of　the　high-pressure　fuel　line　and　delaying　the　time　for　injector　needle　seating　to　cut　off　fuel　injection.　There　are　two　cavitation　regions:　the　conical　region　and　the　downstream　of　the　conical　region.　Due　to　great　pressure　differential　between　the　inlet　and　outlet　of　the　control　valve,　the　cavitation　in　the　conical　region　induces　choking　flow.　Once　choking　flow　occurs,　the　mass　flow　rate　maintains　stable　even　as　the　pressure　differential　continues　rising,　but　the　discharge　coefficient　(C-d)　declines.　This　choking　flow　increases　the　time　delay　of　stop　fuel　injection.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.