The spindle shaft of heavy load deflection angle milling head is an important functional component of 5-axis CNC machine tool, which has characteristics of high torque and high power, used for large and complex free-form surface processing. The spindle shaft is modeled as Timoshenko’s beam by including the centrifugal force and gyroscopic effects. The effects on inherent characteristic of the spindle shaft of heavy load deflection angle milling head are analyzed owing to axial force and spindle shaft speed. Simultaneously, numerical solution of spindle shaft is gotten. And, the numerical results are well agree with the exact modeling, which illustrates that the theoretical modeling of spindle shaft is correct and the exact dynamic characteristics of spindle shaft is obtained. The analytical modeling results of spindle shaft are effective and helpful for design and improvement of spindle shaft of heavy load deflection angle milling head.