As　an　important　part　of　the　common-rail　(CR)　fuel　system　for　diesel　engines,　the　injector　circulation　capacity　and　the　fuel　injection　mass　flow　rate　vary　with　carbon　deposition　and　wear,　affecting　the　engine　output　performance.　This　study　proposes　a　method　to　identify　the　fuel　injection　rate　online,　based　on　the　rail　pressure　fluctuation　characteristics　induced　by　fuel　injection.　The　control　algorithm　uses　the　signal　from　the　existing　rail　pressure　sensor;　the　diesel　engine　does　not　require　modification　or　additional　sensors.　A　quasi-dimensional　model　of　the　CR　fuel　system　was　built　to　analyse　the　rail　pressure　wave　fluctuation　characteristics,　and　a　parameter　K　was　defined　as　the　pressure　drop　rate.　Based　on　K,　a　control　algorithm　was　proposed.　A　high-pressure　fuel　pump　test　rig　was　built　to　test　the　fuel　injection　performance　under　different　operating　conditions,　and　the　experimental　data　were　processed　by　wavelet　transform.　From　the　test　data,　the　K　of　the　CR　system　was　analysed　using　the　feedback　of　the　rail　pressure　sensor.　The　experimental　results　show　that　the　value　of　K　increases　with　an　increase　in　the　initial　pressure　and　injection　pulse,　and　is　independent　of　the　injection　mode.　The　algorithm　is　feasible,　and　works　more　accurately　with　a　longer　injection　pulse　and　a　lower　pump　speed.　This　method　uses　the　existing　rail　pressure　sensor,　does　not　incur　extra　cost　and　has　great　potential　for　improving　the　injection　accuracy.