Abstract: In order to address technical difficulties in reaming after drilling in directional wells,such as high torque,ease in breakage of reaming tools,long reaming time and poor reaming result,study and test of reaming while directional drilling were carried out.Through analysis of features of reaming tools at home and abroad as well as field application,CSDR bi-center bit was selected as the reaming tool,matched with 475K4570 adjustable mud motor providing force.The nozzle combination and hydraulic parameters were optimized according to the principle of rational distribution of hydraulic power.In the Well THA,successful reaming while directional drilling was realized in China for the first time,at the curve section from an inclination of 14.13°to 42.50°,the reaming section is smooth,with an average rate of over-all angle change of 5.554°/30m and an average hole size as 173.99 mm,saving 50% of the reaming time.Field test shows bi-center bit,matched with PDM,can realize reaming while directional drilling in deep slim hole.Good reaming effect and short reaming time have made it become an effective means of reaming in directional well section.