The　spray　tip　penetration　is　an　important　parameter　to　evaluate　the　fuel　injection　quality.　This　study　presents　an　experimental　investigation　on　the　high-pressure　diesel　injection　process　of　a　common-rail　injector　under　condition　of　varying　pressures,　particularly　on　the　effect　of　the　dynamic　injection　behavior　on　the　fuel　spray　penetration.　The　injector　delivery　process　is　tested　on　a　common-rail　test　rig,　with　an　optical　test　rig　to　examine　the　diesel　spray　based　on　a　constant-volume　bomb　(CVB)　system.　This　process　includes　two　stages:　i.e.,　the　opening　stage　and　the　keeping　stage,　while　the　two-stage　injection　momentum　causes　crucial　influence　on　the　fuel　spray.　Correspondingly,　the　fuel　spray　penetration　also　has　two　stages.　In　Stage　I　(the　dynamic　stage),　the　spray　penetration　sharply　increases.　In　Stage　II　(stable　stage),　the　spray　penetration　stably　increases.　When　the　injection　pressure　rises,　the　spray　tip　penetration　increases,　and　the　two-stage　turning-point　time　is　shorter.　Considering　this　two-stage　characteristic,　an　equation　is　proposed　to　predicate　the　spray　tip　penetration:　it　is　based　on　the　dynamic　injection　pressure　for　the　dynamic　period　but　the　constant　pressure　differential　for　the　stable　period.　The　predicated　spray　tip　penetration　has　good　agreement　with　the　test　data.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.