Compared with local spatial optical solitons and linearly propagating beams, nonlocal spatial optical solitons each have a large phase shift during their propagation. However no one has paid attention to the intrinsic characteristics since the theoretical proof by Guo. We develop the phenomenological theory of Guo and find that there appears π phase shift with the change of the soliton power or the power of the pump soliton. Based on the conclusion that the modulation of the pump-soliton power on the signal-soliton phase has a high sensitivity, we propose a feasible scheme of realizing the optical switch.