The invention discloses an outdoor lamp mounting base plate. The base plate comprises wiring terminals and a conductive plate; the wiring terminals are connected with the conductive plate; a sliding groove is formed in the bottom of the conductive plate; and a sliding plate is arranged in the sliding groove, makes contact with the bottom surface of the conductive plate, conducts electricity and isconnected with a lamp. During mounting of the outdoor lamp mounting base plate, the top surface of a casing can fit the wall surface or the ceiling surface of a building through screws to be fixed, the two wiring terminals are connected with a power supply, then a wire of the lamp is stably electrically connected with a mounting hole in a lamp mounting plate, and the lamp can be stably electrically connected with the power supply. The lamp and the lamp mounting plate can be pushed to move back and forth, a connecting rod and the sliding plate can move back and forth in the sliding groove, andthe sliding plate fits the conductive plate all the time, so that the lamp is kept electrified all the time. By the aid of the outdoor lamp mounting base plate, the lamp can be mounted on the outdoorbuilding and can be moved, the electric power is saved, and the lighting range of the lamp can be enlarged.