Combined with external modulation and stimulated Brillouin scattering(SBS) amplification technology, a new method of generating triangular waveform signal by synthetizing signal envelopes with proper time delay is proposed and demonstrated. In this scheme, the continuous light emitted by lasers is divided into two branches. One is modulated by Mach-Zehnder modulator and the fundamental frequency modulation signal, one third of the Stokes frequency shift, is obtained. The other one is used as SBS pump light to amplify the positive 3rd-order sideband of the modulation signal, and a frequency tripling harmonic signal is obtained. After independently controlling the phase and power of envelopes of the two harmonic signals, a triangular waveform signal with good approximation is generated by synthetizing these two signal envelopes with proper time delay. By changing the wavelength of continuous wave, the frequency of the generated triangular waveform signal can be tuned in a certain range. The feasibility of this scheme is proved in theoretical simulation and experiment. The scheme avoids complex spectral lines manipulation and the system stability is improved. It provides a new idea for arbitrary waveform generation technology.