Abstract: Through PWD, surface computer control system, back pressure pump and choke etc., delicate controlled pressure drilling system could adjust the wellhead back pressure and delicately controls the bottom hole pressure to meet the needs of safe and fast drilling. The important index reflecting the performance level of delicate controlled pressure drilling system is the response time. Short time can quickly balance off bottom hole pressure variation and keep drilling basically under constant bottom-hole pressure. The pressure controlling response time is consisted of three parts: PWD signal transmission time, the time from analyzing signal to adjusting wellhead back pressure by surface control system and the time of back pressure propagating from wellhead to bottom. This paper regarded the third part—the time of back pressure propagating from wellhead to bottom, as the research object. The computational mode of back pressure propagation velocity in annular was established, and the main factors affecting the velocity were analyzed, which were drilling fluid density, gas content and cutting content. Field data were used to calculate and analyze the pressure controlling response time. Finally theoretical supports were provided for delicate controlled pressure drilling.