Abstract: In deepwater well control,gas exchange effect might happen when invading gas from formation flows into long and thin choke lines,which can affect the value of choke pressure significantly and increase the difficulty of well control.Therefore,it is very important to study the issue.The gas exchange effect occurring in choke lines and the choke pressure change caused by this effect were studied from different aspacts with numerical simulation and case study based on multiphase flow model.The simulation indicates that gas volume fraction in choke lines varies largely and rapidly and gas exchange effect is obvious during deepwater well killing;the value of choke pressure decrease as circulation volume increases and the changes of this value rise with the increase of circulation volume;choke pressure goes up with the rise of pit gain;choke pressure of standby is larger than the choke pressure while drilling,and choke pressure while tripping was higher than that of standby;the choke pressure during deepwater well killing would change faster than that onshore;the smaller the choke line,the lower the initial choke pressure,the higher the peak value and the larger variation of the choke pressure,and the more apparent the gas exchange effect in choke line.