An improved order analysis method based on rotate speed adjustment is put forward and applied to extract the weak fault characteristics of automobile transmission gear. The localization of traditional order analysis method in analyzing the fluctuating rotate speed signals is analyzed theoretically, and the improved order analysis method based on rotate speed adjustment is put forward. The traditonal and improved order analysis method is adopted to analyze the fluctuating rotate speed signal respectively, and the comparison of the results shows that traditional order analysis method is susceptible to the impact of rotate speed fluctuation, which can be conquered by the improved order analysis method effectively, the analysis effect is ameliorated and the precision is improved evidently. The accelerating process’s vibration signal of automobile transmission is analyzed by traditonal and improved order analysis method respectively, and the results show that normal and light fault state of the gear can not be distinguished by traditional order spectrum correctly, but that can be identified by the improved order spectrum clearly, so the signal with fluctuating rotate speed can be analyzed by the improved order effectively and the weak fault characteristics can be extracted correctly.