Abstract: Equilibrium equations on nodes are taken as constraint conditions, and Lagrange multipliers are substituted into complementary functionals expressed with base forces, and derivatives of displacement with respect to coordinates.Then a conditional stationary problem is transformed into an unconditional stationary problem.Based on this complementary energy principle, internal forces and displacements are obtained when a rod system is in large deformation.Thus it is feasible for the complementary energy principle to apply to large deformations in rods systems.