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.