Due to the limitations of self-structure and test conditions, the actual unloading point is difficult to coincide with the theoretical centroids in a mechanically steerable satellite antenna’s ground zero gravity simulation test. There is a residual moment of unloading. Three-axis mechanical steerable antenna was taken as the research object. Based on a suspension system, the static models of antenna unloading under vertical and horizontal deployment modes were established. The general formula of the joint residual moment was derived. The influence of deployment mode on the joint torsional moment under different working stages was studied. The result shows that the joint torque in the horizontal deployment test is much better than that in the vertical deployment test, with low data peaks and good stability. The horizontal deployment test is preferred for the ground test. The method has been successfully applied to the antenna development of multiple satellites. It has practical guidance and application value for subsequent similar antenna unloading scheme designs.