Abstract: Reverse circulation well killing method is commonly used in offshore tubing conveyed perforating (TCP). With this method, the lag time is short, so that the time for killing well can be saved; the formation fluid does not pollute the casing annulus, so that the wellbore can be kept clean. According to the characteristics of offshore perforating string, we deduce the formula for calculating the densities of overflow and well killing fluid, as well as the calculation formula of shut-in pressure and circulating pressure in the well killing process. Calculation is carried out for a reverse circulation killing example. The results show that the casing pressure is the highest at the beginning, and after the overflow all goes into the drilling tool, the initial circulating pressure is lowered to the stable circulating pressure. When the well killing fluid enters the annular vacuum and reaches the negative pressure valve, the casing pressure drops to the terminal circulating pressure. During the rising and discharging of the overflow or the well killing fluid, the casing pressure remains unchanged.