Control　valve　is　one　critical　component　of　electronic　unit　pump　fuel　system.　Control　valve　controls　the　pressure　build-up　within　the　high-pressure　fuel　line　through　rapid　opening,　which　is　a　dynamic　process.　On　the　basis　of　theoretical　analysis,　there　are　two　key　reasons　for　the　unsteady　flow　within　the　control　valve:one　is　the　acceleration　of　fuel　which　is　induced　by　the　valve　needle＇s　dynamic　movement,　and　another　is　the　acceleration　of　fuel　flow　due　to　the　change　of　the　inlet/outlet　pressure,　and　this　acceleration　of　fuel　flow　enhances　the　inertia　force.　Comparison　of　the　flow　field　characteristics　of　the　control　valve　are　made　under　both　steady　and　dynamic　conditions　by　simulation.　Results　show　that　with　the　increase　of　valve　opening,　the　cavitation　becomes　stronger　under　the　unsteady　condition　but　cavitation　declines　under　the　steady　condition.　There　are　significant　differences　between　steady　and　unsteady　conditions　on　the　flow　velocity　during　the　whole　opening　process.　In　small　valve　needle　opening,　the　flow　capacity　of　steady　condition　is　higher　than　that　of　unsteady　condition;however,　with　the　increase　of　valve　opening,　the　flow　capacity　of　unsteady　become　greater　than　that　of　steady　flow.　©　2016,　Editorial　Office　of　the　Transaction　of　CSICE.　All　right　reserved.