The　control　valve　determines　the　fuel　offloading　process　of　the　high-pressure　fuel　system　in　diesel　engines.　During　the　fuel　offloading　process,　when　the　control　valve　instantaneously　opens,　the　inlet　pressures　and　the　outlet　pressures　of　the　valve　change　quickly,　and　the　inner　flow　of　the　control　valve　is　transient　and　unsteady.　This　study　presents　an　investigation　into　the　flow　characteristics　in　the　control　valve　region　during　the　fuel-offloaded　process　of　a　diesel　unit　pump　fuel　system,　studies　the　reasons　for　unsteady-flow　conditions　and　further　analyses　the　effects　on　the　flow　capacity　of　the　valve　based　on　the　computational　fluid　dynamics　method.　The　results　reveal　that　the　key　reason　for　unsteady　flow　in　the　control　valve　is　the　dynamic　movement　of　the　valve　needle,　which　induces　changes　in　the　inlet　pressure　and　the　outlet　pressure　so　that　the　fluid　in　the　conical　region　accelerates.　In　comparison　with　steady　flow,　unsteady　flow　in　the　control　valve　has　different　pressures　in　the　conical　region　and　at　the　conical　outlet,　different　cavitation　intensities　and　different　locations　of　the　bubbles.　Moreover,　the　differences　between　unsteady　flow　and　steady　flow　become　greater,　particularly　for　small　valve　needle　openings.　Unsteady　flow　in　the　control　valve　greatly　influences　the　fuel　offloading　process　of　the　unit　pump　fuel　system.