The invention provides an automatic loss-reducing and electricity-saving device for controlling a streetlamp transformer, which consists of a high/low voltage switch controller of the transformer and a power controller, wherein the device starts or stops the streetlamp transformer according to a set operation requirement sequence, namely, the transformer is automatically operated and supplies power to a streetlamp at night, and the streetlamp is powered off and then the transformer is powered off after dawn, so a high-voltage switch is operated without load current, a disconnecting switch or a combinative electric appliance of the disconnecting switch and a fuse can be selected and used and the investment cost of equipment is reduced. The electricity-saving device can power off the transformer in a no-load state for more than ten hours during the daylight so as to reduce unnecessary electric energy consumption, slow down the insulation aging of the transformer, prolong the service life of the transformer, save the engineering cost of the streetlamp and have wide market prospect and application value.