Global behavior of complex nonlinear gear system is a method for gear fault diagnostic. The simple cell mapping method, which is a numerical computing technology to solve the global behavior of complex nonlinear system, is used to global analysis of the dynamical behavior of nonlinear gear system under different degrees of wear faults. By transforming the state space of the nonlinear gear system into the cell space and comparing the domains of attraction and cell of attraction under different degrees of wear faults, the nonlinear system dynamic global developing rule of the gear wear fault is obtained, which offers a new method for the nonlinear gear system fault diagnosis.