Abstract: During under-balanced drilling, air drilling or foam drilling, such problems as uncertain downhole pressure, large errors on formation pressure prediction, improper control of the under-balanced downhole pressure may often occur when there are multi-phase flows in drilling fluid. Field test was conducted to accurately monitor the downhole pressure when there are multi-phase flows during under-balanced drilling in the third section, and the results proved that the conventional theoretical error is about 10%, sometimes as high as 13% when there is gas cut. A new method is presented to accurately calculate formation pressure by using the real monitored downhole pressure data, while the downhole under-balanced pressure and drilling fluid average density in the annular can be calculated also, thus a better determination of the mud weight in the following under-balanced drilling becomes possible.