In order to break through the limitation of Electronic analog-to-digital converter in speed and time-delay precision,a wideband digital frequency measurement method based on optical sub-sampling is proposed.Taking advantage of ultrashort optical pulse from passive mode-locked fiber laser,multi-wavelength optical pulse cluster with controllable time-delay is produced.Traveling through electric-optic modulator,pulse cluster samples the signal and is detected by photodiode array,and then we could calculate the frequency spectrum in electric domain.Combined with Multiple Signal Classification algorithm (MUSIC),the method has advantage of high sampling bandwidth,accurate time-delay and Wavelength Division Multiplex (WDM) technology.According to theoretical research and numerical simulation result,this method is able to get high-resolution multiple frequency measurement with a bandwidth of 20GHz.