The invention discloses an ultrafast optical wavelength measuring system. The ultrafast optical wavelength measuring system comprises a narrow linewidth optical maser, a first optical coupler, a second optical coupler, a third optical coupler, a fourth optical coupler, a fifth optical coupler, a mode-locked optical maser, a first photoelectric detector, a second photoelectric detector, a third photoelectric detector, a fourth photoelectric detector, a fifth photoelectric detector, a first analog-digital conversion module, a second analog-digital conversion module, a digital signal processing module and a dispersion compensation fiber. According to the ultrafast optical wavelength measuring system, a wavelength of a measuring optical signal is obtained by processing an output wide spectrumoptical signal and a reference optical signal of the mode-locked optical maser and a beat frequency signal of an optical signal to be measured, and the measuring range is equal to the spectral width of the mode-locked optical maser, namely, 10-100nm magnitude. The measuring rate is equal to the pulse repetition frequency of the mode-locked optical maser and can reach 50 MHz. A cross-correlation algorithm is introduced in the digital signal processing module to improve the measurement accuracy, and the measurement accuracy is inversely proportional to the bandwidths of the photoelectric detectors multiplied by a dispersion coefficient of the dispersion compensation fiber, namely, 10pm magnitude. Therefore, the ultrafast optical wavelength measuring system can realize optical wavelength measurement with a large bandwidth, a high rate and high accuracy.