Abstract: Dynamic kill drilling technology can effectively handle the well kick, loss of circulation or wellbore collapse during surface drilling of deep water. In order to figure out the pressure variation characteristics of dynamic kill drilling in deep water, this paper established a physical model of wellbore in dynamic kill drilling in combination with the basic theory of dynamic kill drilling. By setting up the initial flow rate and change rates of seawater flow rate and weighted drilling fluid flow rate, it derived the wellbore pressure computation model in variable flow rate and variable density scheme. According to the field data of Mexico Gulf deep water drilling, it calculated the annualar density, annualar pressure, annular pressure loss, and bottom hole pressure during dynamic kill drilling. The results demonstrated that the key step of dynamic kill drilling technology is controlling the mixture density by regulating the flow rate of sea water and weighted drilling fluid in timely, therefore, the annular fluid column pressure in drilling process is controlled, which is favorable to safety drilling in surface of deep water. Rate of penetration (ROP) has a great effect on bottom hole pressure in dynamic kill drilling. The additional density generated by cuttings increases with the increase of ROP, causing a larger bottom hole pressure.