We propose a technique to separate the upper and the lower sidebands of an optical carrier based on orthogonal optical coherent receiving, which can be used in Brillouin scattering distributed fiber-optic sensing systems to separate Stokes and anti-Stokes scattering without loss. In the orthogonal optical coherent receiving technology, phase information of the optical field is preserved. So the upper and the lower sidebands can be arranged in coherent addition or cancellation states in different outputs, and the separation of the upper and the lower sidebands is realized. The results show that the technique can output the upper and lower sidebands from different ports, with a good linear relationship between input power and output power. The crosstalk between the separated upper and lower sidebands can be less than -20 dB. Compared with frequently employed optical filtering, this technique has no temperature sensitive component and shows good stability and robustness.