Abstract: In order to reveal flow behavior of drilling fluid in drill string,numerical simulation method is adopted to establish a mathematical flow model about Herschel-Bulkely fluid in whirl drill string.Combined with the theory of fluid mechanics,the model was used to analyze and compare velocity and apparent viscosity profiles.The effects of variation in drill string rotation velocity,revolution velocity,axial flow velocity and fluid density on flow field were examined,the regularity of effect with each parameter on friction pressure loss in whirl drill string was found.The study shows that friction pressure loss was reduced with the positive increase or reverse decrease of revolution speed,but friction pressure loss increased with the increase of rotation speed.Therefore,when taking friction pressure loss into account,the description of drilling fluid flow in whirl drill string will be much closer to actual drilling condition,facilitating the optimization of drilling circulation system,it can be of great significance to safe and efficient drilling.