Embodiments of the present invention provide a variable optical attenuator. The variable optical attenuator includes: a collimator, a switchable polarization grating, a reflector, and a voltage controller for adjusting a voltage between electrodes at both ends of a liquid crystal layer of the switchable polarization grating, where the collimator, the switchable polarization grating, and the reflector are disposed successively; the collimator is configured to receive incident light and output the incident light to the switchable polarization grating; the switchable polarization grating is configured to diffract the incident light for one time and then perform emission onto the reflector; the switchable polarization grating is further configured to diffract, for one time, a beam reflected back by the reflector, and then emit resulting diffracted light; and the collimator is further configured to receive the diffracted light and output the diffracted light.