Scale model test is an effective approach to obtain mechanical characteristics or response of large composite spatial frame, in which the design of scale model is one of the most important parts. Large composite spatial frame was taken as the object. The limitation of the existing traditional methods was elaborated; based on the discrete similarity, the equational analysis method combing the finite element method was proposed; the similarity relationship for static response and dynamic characteristics of large composite spatial frame was deduced by this method; the 1/5 scale model of a composite keel was designed by the proposed method and was verified through experiments. The verification result shows that the proposed method and the deduced similarity relationships can be used to guide the scale model test of large composite spatial frame.