The invention provides a microcomputer controlled street lamp transformer loss reducing and power saving automatic device which comprises a high-voltage switch controller and a low-voltage switch controller of a transformer, and a power supply controller; the device controls the work and the stop of the control street lamp transformer according to a set order, i.e. in the nighttime, first the transformer is automatically started, and then supplies power to a street lamp; and after dawn, the street lamp is powered off, and then the transformer is powered off, so that the high-voltage switch isoperated without load current, and an isolation switch or an electric appliance combining the isolation switch and a fuse can be selected, so as to reduce the invention cost of the equipment. The power-saving device can lead the transformer to be power off without load in more than 10 hours in the daytime, thereby reducing the unnecessary electric energy consumption of a power grid, and simultaneously, mitigating the insulation aging of the transformer, prolonging the service life of the transformer and saving the engineering cost of the street lamp, and has very good market prospect and application value.