Applying similarity theory to study the characteristics of scale model for getting agreement between scale model test and prototype test. On the basis of the theoretical analysis of scale model test, a formula derivation of similarity condition was carried out based on similarity theory. A numerical example for joint of steel beam to CFST (concrete-filled steel tubular) was given to show the accuracy formula of similarity condition. The result showed that error results from non-full-similar geometrical properties was considerable between poorly-proportioned model test and prototype. For getting good agreement between scale model test and prototype test, the well-proportioned model should be calculated by the formula of similarity condition for the joint of beam to column.