Combined gear drive is a common transmission form for high power drive systems. In order to study its behaviors of redundant drive, which improve the system reliability but lead to a problem of inter-excitation of vibrations, dynamic analysis becomes very necessary. First, considering the influences of gear meshing errors, backlashes and time-varying meshing stiffness, a nonlinear dynamic model of sixteen degrees of freedom is developed by using lumped parameter method. Then a nonlinear differential equation set is derived for calculating the steady-state forced response to the internal sinusoidal excitation. After that, time history wave of displacement responses, phase graphs, Poincaré maps and FFT (Fast Fourier Transform) spectra are analyzed. Finally, some useful conclusions are obtained.