Dynamic displacement is an unconventional well killing method in which killing fluid is injected and gas is discharged simultaneously. The determination of the maximum injection rate of killing fluid and the control of casing pressure of wellhead are the key to the success of well killing. In this paper, a mathematical model of the dynamic displacement method is established based on the fact that the bottom hole pressure is maintained to be constant during the killing fluid injection process, the mass conservation is satisfied during the gas discharge process, and the modified Drosos fluid generic correlation is satisfied during the fall process of the killing fluid. By comparing the calculated maximum injection rate of killing fluid with the Ramtahal experimental data, the error is only 7. 0% and 0. 1 % when the casing inner diameter is 152 mm without drilling string in the casing and the drilling string outer diameter is 51 mm, which can be applied to the calculation of killing fluid by dynamic displacement method. The model is used to analyze the effect of different factors on the maximum injection displacement of killing fluid and the influence of casing pressure on wellhead. The results show that the maximum killing fluid injection rate goes up with the drill string eccentricity and surface tension, which means the casing pressure is decreased faster and the less killing time is needed. On the other side, the maximum killing fluid injection rate goes down with gas volume and viscosity, which means the casing pressure is decreased slower and more killing time is needed.