The free space positioning method is to be implemented by a free space positioning system that includes a light source module, an image capturing module disposed at the object, and a computing module. The computing module includes a processing unit and a storage unit. The light source module includes light sources O, X, Y that cooperate to define a plane, and a light source Z disposed at one side of the plane. The storage unit stores a plurality of light source orientation data and a plurality of pre-stored attitude angles respectively corresponding to the light source orientation data. The free space positioning method comprises the steps of: (A) capturing an image of the light source module by the image capturing module to generate a to-be-judged image; (B) analyzing the to-be-judged image to obtain to-be-judged information; (C) screening light source orientation information similar to the to-be-judged information; and (D) estimating an attitude angle of the object according to a pre-stored attitude angle corresponding to each of the light source orientation information.