Abstract: The wellbore trajectory control tool is a sort of guiding drilling tool to achieve the orientation function while drilling. Under complex conditions, the spindle is under the actions of the bit pressure, the torque, the force of biasing mechanism and so on. This paper studies the mechanical behavior of the prototype spindle. A spindle mechanical model is established. With the model, the influences of the housing stiffness and the position of the eccentric mechanism of the spindle on the mechanical behavior and the deflecting capacity are analyzed. The data analysis shows that the deflection angle of the lower end of the principal axis, the force of the biasing mechanism and the maximum bending moment will increase linearly with the eccentric displacement of the eccentric ring. With the increase of the housing stiffness and the axial bearing distance from the mounting position of the biasing mechanism, the tool deflecting capacity will be enhanced and the maximum bending moment of the spindle will increase. It is shown that, we have selected the most suitable house stiffness and the mounting position of the biasing mechanism, which would serve as a reference for the spindle design and the improvement of the deflecting capacity of the wellbore trajectory control tool.