The invention discloses a lower cover of an energy-saving lamp with the function of positioning a lighting tube. A pair of lighting tube mounting holes is formed on a cover body, and a pair of semicircle positioning ribs concentric with the lighting tube mounting holes is arranged around a first lighting tube mounting hole on the inner surface of the cover body. In the structure of the plastic lower cover of the invention, the centers of the two semicircle positioning ribs are coaxial with the lighting tube holes; since the lighting tube positioning ribs are semicircle, the area contacting the lighting tube is large, the function of fixing the lighting tube can be well performed, and the lighting tube is not easy to be detached from the plastic, so that the lighting tube can be well positioned. In the invention, an upper cover can enable the plastic to fix the lighting tube as long as the lighting tube is inserted into the plastic, and then the invention can enter a gluing procedure. Therefore, the invention is simple in operation, no mould is needed, the manufacturing cost is lower, and the production efficiency is high.