Abstract: In view of gas slip in the mud and gas weight, the formulas of calculating dynamic BHP of gas-cutting mud and computer programs have been developed in this paper. The new method has been compared with that one generally used in the past in case the gas slip and gas weight were neglected. From this a conclusion can be drawn that when the well depth is within 2000 to 4000 meters, gas slip has an obvious effect on BHP. The abs olute error may be up to 1.92 MPa, and the relative error 5.8%. Gas weight also has some influence on BHP,the absolute error may be up to 0.77MPa and the relative error 2.3%.

Key words: Gas-cutting Mud, Bottom Hole Pressure （ BHP ）, Drilling Control