The dynamic reflection spectra of amorphous blue phase III were investigated. When an electric field is applied to a blue phase III cell, the reflected wavelength does not shift obviously, but the intensity of reflection increases or decreases in a few ms. This fast intensity-tunable phenomenon in blue phase III relates to the dielectric anisotropies and chiralities of the liquid crystal and can be explained by the double twist model consisting of randomly orientated double-twisted cylinders. This study shows that blue phase III can act as a fast intensity-tunable reflector for a specific wavelength. V