The　design　and　synthesis　of　g-C3N4　with　favourable　physical-chemical　architecture　are　important　factors　in　influencing　its　photoactivity.　Herein,　a　phosphorus　(P)　vapor　assisted　synthetic　strategy　was　employed　to　fabricate　the　P-doped　hollow　g-C3N4　photocatalysts　with　ultrathin　shell　structure　(＜　25　nm　in　thickness).　The　doping　of　P　and　the　compression　of　shell　thickness　of　the　hollow　g-C3N4　were　induced　simultaneously　by　the　P　vapor　during　the　heating　process.　The　as-prepared　P-doped　ultrathin　hollow　g-C3N4　sphere　(P/UH-CNS)　photocatalysts　exhibited　enlarged　surface　area　and　light　responsive　range,　and　improved　charge　transportation　efficiency　by　suppressing　the　charge　recombination　and　self-trapping　within　g-C3N4.　These　resulted　in　a　high　photocatalytic　hydrogen　evolution　rate　of　9653　mu　mol　h-1　g-1.　The　charge　dynamics　in　this　P/UH-CNS　system　was　revealed　in　detail　by　using　the　ultrafast　time-resolved　spectroscopy.