According to the restriction of self-propelled gun full scale test, and because the traditional vibration test can not be used for non-contact, non-disassembly and nondestructive test, a method that utilizes the transient acoustic signal for gearbox fault diagnosis was introduced. This paper presented an order cepstrum analysis, a novel method which combined the order analysis and cepstrum analysis together, to conquer the disadvantage of traditional FFT when processing non stationary signals, the result shows that it can solve the “frequency ambiguity” efficiently. Compared with the vibration signals, the gearbox gear wearing fault experiment based on transient acoustic signals shows its feasibility and superiority.