A photonic approach is proposed to generate microwave arbitrary waveforms based on the stimulated Brillouin scattering (SBS) effect and the Sagnac loop. The Sagnac loop is utilized to generate the optical signals of ±1st-order and ±3rd-order sidebands. Due to the principle of velocity adaptation, even order optical sidebands are suppressed. Thanks to the SBS effect, the microwave signal consisting of odd order harmonics is generated after the photodetector (PD), which can construct the triangular waveform, the square waveform, or the sawtooth waveform by adjusting the weight of the harmonics.