The optimal control problem of a class of nonlinear discrete-time singularly perturbed systems is studied. A composite optimal controller is designed based on the decomposition of the slow and fast subsystems, and an original high- order optimal controller is developed. Then, the relationship between the composite controller and the original controller is analyzed. It is proved that the composite control input is ??(??) close to the original control input under some conditions. The result is verified by two simulation examples.