Abstract: A precise background theory of computational mechanics is formed. Saint-Venant’s principle is discussed in chain model by means of this precise theory. The classical continued fraction is developed into operator continued fraction to be the constrictive formulation of the chain model. The decay of effect of a self-equilibrated system of forces in chain model is decided by the convergence of operator continued fraction, so the reasonable part of Saint-Venant’s principle is described as the convergence of operator continued fraction. In case of divergence the effect of a self-equilibrated system of forces may be non-zero at even infinite distant sections, so Saint-Venant’s principle is not a common principle.