Due to the slim hole at the lower part of the ultra-deep and deep wells,the eccentricity and rotation of drill string and drilling fluid properties have great effects on the annular pressure drop.This leads to the fact that conventional computational models for predicting circulating pressure drop are inapplicable to hydraulics design of deep wells.With the adoption of helical flow theory and H-B rheological model,a computational model of velocity and pressure drop of non-Newtonian fluid flow in the eccentric annulus was established for the cases where the drill string rotates.The effects of eccentricity,rotation of the drill string and the dimensions of annulus on pressure drop in the annulus were analyzed.Drilling hydraulics was given for an ultra-deep well.The results show that the annular pressure drop decreases with an increase in eccentricity and rotary speed,and increases with a decrease in annular flow area.There is a great difference between static mud density and equivalent circulating density during deep well drilling.