Abstract: Fore the stress concentration,the main failure while drilling is intermediate belt,especially in dog-leg section,with the change of load,it is easy to form of fatigue crack,and caused drill pipe damaged or even fault.Used finite-element software simulated the stress of drill pipe,and can direct the design for thicken of drill pipe.Mechanics analysis shows that decrease diameter of inner-bore,increase length of inner-cone area,increase radius of curvature of inner-cone are all can decrease index of stress concentration.But the available structure is not adequate to decrease stress,so it is feasible to decrease the stress concentration index by alter the structure of thickened intermediate belt.