Long shaft is easy to be bended and deformed in cold extrusion. Taking account of error in calibrating strap of concave die, the finite element model was built. And some factors affecting bending, such as relative length of calibrating strap and entrance half angle and diameter of steel billet were analyzed. The simulation results show that both increasing in relative length of calibrating strap and entrance half angle of concave die, and reducing in diameter of steel billet contribute to reduce bending deformation caused by error of calibrating strap. The process experiment proves that optimized process parameters are effective to reduce bending deformation of shaft and improve the forming quality. The numerical simulation results are helpful to manufacture.