The utility model provides an electricity saving device for a street lamp, which consists of a transformer high-voltage switch controller, a low-voltage switch controller and a power supply controller, wherein the power supply controller is respectively connected with a high-voltage switch controller and the low-voltage switch controller, the high-voltage switch controller and the low-voltage switch controller are respectively connected with a high-voltage switch and a low-voltage switch, the device controls the on-off state of a street lamp transformer according to the set sequence, i.e. at night, the high-voltage switch of the transformer is switched on for operation, and then, the low-voltage switch is switched on for supplying electricity to the street lamp; after the day breaks, the low-voltage switch is firstly switched off for cutting off the power supply of the street lamp, finally, the high-voltage switch is switched off for cutting off the power supply of the transformer, and the electric energy consumption of an electricity network is reduced; and in order to realize electricity saving, two switches arranged at the low-voltage side of the transformer respectively control two sides of one road or the crossed control is realized every one street lamp, so the power failure time of one path of street lamps can be brought forward to 24 o'clock, i.e. a part of street lamps are reduced after midnight when people and vehicles are few to reach the goal of electricity saving. When being applied, the electricity saving device has wide market prospects.