The invention provides a preparation method of a magnetic nano ink and a magnetic flexible circuit or device. The magnetic nano ink comprises magnetic nano particles. The surface of the magnetic nano particles is covered by a layer of silver. The thickness of the silver layer is 1 to 50 nm. According to the technical scheme, the surface of magnetic nano particles is covered by a silver layer; and when the thickness of the silver cover is 1-50 nm, the sintering temperature is effectively reduced. After the ink is printed or manually painted on a flexible substrate, low temperature sintering (20-100 DEG C) can be realized. The ink prepared from the silver layer covered magnetic nano particles has a good flowing property, can be sintered at a low temperature to form electrodes and magneto-electric devices on a flexible substrate, can be used to prepare high sensitivity flexible magneto-electric devices, can realize stable resistivity under a bending condition, and can respond to a non-contact type magnetic field.