Abstract: Based on the analogy principle between structural mechanics and optimal control, the multi-level substructure method in structural mechanics is used for the computation of the constrained nonlinear control system. The nonlinear system and its constraint are linearized first, and the definition of the time-interval mixed-energy is given, then the condensation formulas of the time-inter val are obtained according to variational principle. Finally the iterative algorithm is established for the solution of the constrained nonlinear control system, the corresponding numerical example is given.