A plant light supplementing lamp is provided. The plant light supplementing lamp includes an electronic ballast, a mounting bracket, a lamp support, lamp holders and a lamp shade; a number of the lamp holders is two, the lamp holders are respectively located at a top and a bottom of the lamp support, limiting plates are provided on a front side of a top of the electronic ballast, an adapter is clamped on an inner side of the limiting plate, sliding grooves are provided in left and right side surfaces of the adapter, and fixing grooves are provided in corners of the left and right side surfaces of the adapter.