In order to solve the large dynamical system effectively, various model reduction methods have been proposed. By analogy with the nonlinear Galerkin methods,a large dynamical system was split into a 'slowly' subsystem, a 'moderately' subsystem, and a 'quickly' subsystem. Accordingly, an improved nonlinear Galerkin method was developed by slaving the contribution of the moderate subsystem to the slow subsystem during numerical integration. Then, a typical 5-degree-of-freedom system with cubically nonlinear stiffness was given to show the accuracy of the new method.