The invention discloses a power grid isolation power supply method and circuit. The method comprises the following steps: voltages in a power grid are divided to acquire a partial voltage from the power grid, and the partial voltage is used as an input voltage; rectification filtering is carried out, the processed signal is converted into a first oscillating signal, then, the first oscillating signal is converted into an ultrasonic signal, after penetrating through an insulating medium, the ultrasonic signal is converted into a second oscillating signal, and the second oscillating signal is subjected to rectification filtering processing so as to obtain a second direct-current voltage signal. Due to the adoption of the technical scheme, compared with the prior art, according to the method,a high-voltage capacitor in the high-voltage alternating current network is directly applied to voltage reduction, then, the low-voltage alternating current is rectified, filtered and converted intoa high-frequency ultrasonic signal, electric energy conversion and electrical insulated isolation between the high-voltage power grid and the low-voltage power grid are finished between the ultrasonictransmitting and receiving devices, isolation of the ultrasonic energy conversion and isolation of insulating mediums are used in the technology, the cost is low, the size is small, and the weight islight.