In this paper, a scheme for generating sinc-shaped optical Nyquist pulses based on external modulation is proposed. First, five flat optical frequency comb (OFC) lines are generated by a dual-parallel Mach–Zehnder modulator (DP-MZM) for optical carrier phase cancellation interference. Then, the phase-locked OFC is split into two paths, one of which is transmitted to a single-drive Mach–Zehnder modulator (SD-MZM) for the modulation of the even-order side-band suppression, and the other is used to remodulate the signal in order to obtain equally spaced comb lines. Eventually, equal frequency spaced phase-locked 15-line OFCs are generated and extremely narrow over-zero width Nyquist pulses are realized at 2.5 GHz, 5 GHz, 10 GHz and 20 GHz. The root-mean-square error (RMSE) is calculated for the generated Nyquist pulses which enables the verification of the signal quality.