A single component order bispectrum method based on Fractional Fourier Transform(FRFT) is presented and applied to eliminate the intercrossed items created by analysis of multiple components signal by order bispectrum, and the weak fault characteristics of gear box are extracted. The best FRFT transform order is calculated according to the rotate speed signal of the gearbox’s input axis and the gear ratio, and the vibration signal of gearbox’s accelerated running process is analyzed by FRFT with the best FRFT transform order, the order components are separated into single component signals in that FRFT field and analyzed by order bispectrum respectively, then those order bispectrum results are added to make the single component order bispectrum based on FRFT. The experimental results show that the vibration signal of gearbox contains multiple components, if that signal is analyzed by order bispectrum directly, intercrossed items will be obvious and the order and order bispectrum are inaccurate; if that vibration signal is analyzed by the single component order bispectrum based on FRFT, the other components and noise are shielded, the intercrossed items are eliminated, the order bispectrum of the vibration signal is true and accurate, the weak fault characteristics of gearbox are extracted effectively.