The　diagnosis　of　fuel　injection　quantity　plays　an　important　role　in　the　common　rail　fuel　system　of　diesel　engines.　This　paper　researches　a　method　to　diagnose　the　fuel　injection　quantity　by　analyzing　the　signal　of　the　pressure　sensor.　It　was　concluded　that　the　fuel　injection　quantity　causes　the　fuel　pressure　drop　in　the　common　rail.　The　experiment　results　showed　that　the　rail　pressure　drop　has　a　linear　approximation　with　the　injection　quality.　©　2016　Taylor　&　Francis　Group,　London.