The intensity of forward-scattered light is measured by the forward-scattering visibility sensor,which determines the extinction coefficient of the air, then the visibility can be obtained. Based on the described principle and considering the characters of the light scattering in fog, a forward-scattering visibility sensor is constructed. A modified calculation formula between the measured intensity and visibility is given by comparing the intensity measured outside in fog and visibility recorded. Comparison test on the constructed visibility and a foreign one is also carried out outside in fog.