The invention discloses an illuminating device comprising a reflex housing, a light source arranged in the reflex housing and a lamp-chimney for light to get through and sealing the opening of the reflex housing discretely. Particularly, the lamp-chimney is composed of a central lens unit belonging to Fresnel lenses, a first side lens unit and a second side lens unit; the first side lens unit and the second side lens unit outspread from the opposite sides of the central lens unit, by refraction of the central lens unit, the first side lens unit and the second side lens unit to change the course of light, a substantially rectangular light field is formed outside by the light which is sent out from the light source and then travels substantially toward the opening direction after passing through the lamp-chimney.