Abstract: The narrow operation window is quite common in conventional drilling, the dirty performance of mud may result in serious drilling problems, such as differential pipe sticking, blowout and loss simultaneous and hole collapse. It may affect normal drilling, prolong none production and drilling time, and increase cost. Managed Pressure Drilling（MPD）, a new drilling technology, can be used to solve such problems. Combined with the hydraulic model of multiphase flow in annulus, this paper analyzes the effect of such factors as the volume of mud, the volume of gas, ROP, back pressure and well profile on annulus pressure in MPD, aiming at quick and accurate adjustment of the annulus pressure and the bottomhole pressure within a certain range. So the drilling efficiency is enhanced and the cost is reduced. In near future, MPD will be developed with more security and validity.