This paper proposed and demonstrated a scheme to generate frequency-doubled triangular-shaped waveforms based on external modulation and polarization control. After optical carrier suppression (OCS) modulation in a dual-electrode Mach-Zehnder modulator (De-MZM) and optical double sideband (ODSB) modulation in a dual-parallel Mach-Zehnder modulator (DP-MZM), we obtain the optical spectrum with four main sidebands with equal frequency intervals, while maintaining a consistent 9.5 dB amplitude difference between the inner and outer sidebands. Subsequently, polarization control uses an optical interleaver (OI) and a time delay line (TDL) to eliminate the coherent interference of different sidebands. Finally, by applying a 10 GHz radio frequency (RF) signal, a 20 GHz triangular waveform is obtained. The tunability and feasibility of the triangular waveform generation have been also investigated. This proposal provides a feasible method to generate triangular waveform with high repetition rate.