Abstract: Drag/torque prediction is a key technique of drilling an Extended Reach Well successfully.The contact situation between drill pipe and wall of well was not clear because the clearance was ignored in most models.It was assumed that there is an support point every 3 meter between wall of borehole and drillstring,disconnected the drillstring at the support point,according to the residual method,the relationship of corner and moment in one segment was found;based on the condition that the corners of drillstring are same when two segments are adjacent to each other,the moment iterative formulas were found;and base on the bounder condition,the moments could be calculated,which could be used to calculating the support force.The position of drillstring could be repositioned by the direction of support force.So we got a new set of friction and torque calculation formulas and the clearance was not ignored and the contact situation between drill pipe and wall of well is clear,which could be used for determining the proper position of torque reduction sub and drill pipe protection sub.