To achieve precise identification of backlash in gear systems, a new identification method is proposed on the basis of hysteresis backlash model and the describing function method. A Discrete Fourier Transform is used to extract the fundamental components from the system output digital. Then the relationship between the phase angle of describing function and that of the fundamental components is utilized to identify backlash. A sinusoidal excitation with small amplitude and low frequency is applied as the system input, in order to reduce the impact and collision between meshing teeth, highlight the hysteresis effect and improve identification accuracy. A gear transmission system with backlash is considered as a numerical model, simulation results reveal this method can achieve precise backlash identification. Numerical tests with multi-group excitations are conducted, the results reveal that the effects of the selection of amplitude and frequency of excitation on the accuracy of the identification method can't be ignored.