Mechanical analysis is carried out to the deformation of workpiece when turning slender shafts in the normal and reversed direction under two different conditions of clamping. Mathematical models are set up for the calculation of bending deformation resulting from cutting force in condition of normal and reversed turning. An example is given to show that bending deformation and the resulting machining error in conditions of reversed turning is much less than that in normal turning. The result has been verified by experiments. The mechanical model and numerical analysis can be used in the processing design of slender shaft turning.