The utility model discloses a light operated ornamental lamp for a bicycle which is composed of a mercury rotating switch, a light control circuit board, a high-brightness light-emitting diode, and a lampshade. The utility model is arranged on the spokes of the bicycle; an outer conductor in the mercury rotating switch forms a tetrapetalous petaline cylindrical shape; an inner conductor in the mercury rotating switch forms a cylindrical shape; mercury is arranged between the inner and the outer conductors. Because of adopting the mercury rotating switch and the light control circuit board, the autocontrol of a light is not bright when the bicycle is stopped, and bright when the bicycle is ridden; the light is not bright in daytime, and bright at night can be realized to effectively solve the safe problem of riding at night automatically.