Abstract: Attempting to provide a useful tool for the study of mechanism of the vibratory excitation to gear noise, this paper presents a method and the corresponding computative system with the function of spectrum analysis to model the dynamic transmission error (DET) of a single-stage spur gearing which acts as a vibratory excitation to gear noise through processing indirectly the internal exciting force based on the static transmission error (STE) and solving numerically the vibratory differential equation, of which the transmission error (TE) is set as a couple.