An approach to reducing the harmonic distortion in frequency-doubled triangular-shaped pulse train generation is proposed. It requires a dual-parallel Mach–Zehnder modulator followed by a section of dispersive fiber, as the key component. The basic principle is to make the expression of optical intensity approximately equal to the Fourier expansion of idea triangular-shaped waveform. A detailed expression of the final optical intensity is given by theory and then verified by simulation. It is found that the impact of the undesired harmonic distortion is greatly reduced. An approach to reducing the harmonic distortion in frequency-doubled triangular-shaped pulse train generation is. It requires a dual-parallel Mach-Zehnder modulator followed by a section of dispersive fiber, as the key component. The basic principle is to make the expression of optical intensity approximately equal to the Fourier expansion of idea triangular-shaped waveform. A detailed expression of the final optical intensity is given by theory and then verified by simulation. It is found that the impact of the undesired harmonic distortion is greatly reduced.