Based on the orthogonal superposition theorem of three alternating magnetic components, a universal uniform magnetic spin vector is superimposed using tri-axial Helmholtz coils, achieving successive digital control of the orientation, the rotational speed, and the magnetic flux density of the universal magnetic spin vector. For increasing the magnitude and orientation accuracy of the magnetic spin vector, this paper presents a mathematical model associated with the magnitude error and orientation error of the universal magnetic spin vector, along with a double error compensation method for the magnitude and orientation of the universal magnetic spin vector superimposed by three alternating magnetic components. The double error compensation method includes the current magnitude compensation by three different structural coefficients of tri-axial Helmholtz coils and the current phase compensation by two relative phase differences under linear polarization. The results have shown that the double compensation method can increase the magnitude and orientation accuracy of the rotating magnetic vector effectively, which would achieve an accurate posture adjustment and steering control on the capsule robots in curving environment.