The invention relates to a vehicle-mounted mobile device for detecting the visibility of foggy weather, belonging to devices for detecting road traffic environmental information. The visibility value is finally obtained through measuring the attenuation coefficient of light propagation in fog. A light emitting diode is adopted as a light source, and a photosensitive resistor is taken as a photoelectric detector and used for reception. Fog of different concentrations acts on the attenuation effect of light differently, and light signals received by the photoelectric detector are different. The concentration of the current fog, namely the visibility value, is obtained through calculating the received light signals. The fog in a sampling space is closed by using a front-and-rear-door structure so as to shield the influence on detection caused by stray light and airflows of an external environment. The vehicle-mounted mobile device is simple in structure and convenient in use and realizes the function of vehicle-mounted mobile detection of the visibility of the foggy weather.