Abstract: The mechanics and the geometry are inextricably linked. A geometric drawing method can be used to find the form of a triangular prism of tensegrity structure with two parallel end faces, as well as to find the self-equilibrium forces in the triangular prism of tensegrity. The formulae for calculating the force density of self-equilibrium internal forces in triangular prism of tensegrity are derived. The geometric drawing method for form-finding and force-finding is simple in operating, intuitive and adjustable. The method can be implemented in the CAD software, and even can produce drawings on the paper. The formulae of the force density of self-equilibrium internal forces are simple and can be expressed visually. The form finding method is the geometric implementation of the force density method.