The invention relates to a design method for a spindle of a rotary guiding eccentric stabilizer for well drilling. The method obtains a vertical and horizontal bending dynamic deflection equation of the spindle through an energy method, and the equation is shown by a right formula. The equation gives comprehensive consideration to six parameters such as an axial force P, an inclination angle alpha, a vertical uniformly distributed load qsin alpha, the inner diameter ID of the spindle, the outer diameter OD of the spindle, and the rotational speed omega. Simultaneously the invention also provides a testing method for testing whether the spindle deflection meets design requirements. The method analyzes the deflection equation of the spindle of the rotary guiding eccentric stabilizer to obtain the relationship between the dynamic deflection and the static deflection of the spindle under the condition of usual drilling parameters. Furthermore, the method compares the deflection equation under the condition of vertical and horizontal bending with the deflection equation under the condition of plane bending, thereby providing important reference for the design and calculation of the spindle of the rotary guiding eccentric stabilizer.