A warning light used in lane change of a vehicle is provided, in which the light is installed in a side of a rearview mirror housing where the rearview mirror is connected to the vehicle, or the lights are installed in the two sides of the two front doors of the vehicle, so that the lighting emitted by the light is deflected or directed at the driver's eyes in the vehicle. The warning light includes a housing, a light guide, at least one LED light, a circuit board and a mask which is sealed by mold welding disposed on a side of the housing. There are cavities arranged in the rearview mirror or the two sides of the vehicle's two front doors where the light is inserted and is detachably installed in the cavity. A prism in the light guide deflects and directs the emitted light toward the driver's eyes.