The bending stiffness of axis is an important influencing factor in the rotor dynamics. Rotor disc thickness has a great effect on the bending stiffness. The finite element software ANSYS is used to establish one-dimensional and threedimensional models of the traction motor rotor system for railway locomotive. The impact of the disc thickness on the critical speed is also analyzed. With the reduction of the disc thickness and concentration of disk mass,the first critical speed decreases linearly. In addition,when the disk mass is concentrated onto a very thin disk,the critical speed calculated by three-dimensional model is close to the calculation result of one-dimensional beam element method. Based on this way,the application range of a one-dimensional model is then discussed. Finally,unbalanced response analysis of three-dimensional model is given to discuss influence of different disc thickness,unbalance mass and damping ratio on the unbalance response.