The invention provides a magnetic field sensor utilizing the giant magnetoresistive effect of a ferromagnetic nano-ring, and belongs to the technical field of magnetic nano-sensors. The sensor is composed of a ferromagnetic nano-ring prepared on a substrate of nanometer size and a group of lead wires; the group of lead wires connected to the two ends of the ring also serve as a constant current source connecting wire and a voltage measurement connecting wire; the outside diameter D of the ferromagnetic nano-ring is 7 nm-10 um; the inside diameter d is smaller than the outside diameter D; the width of the ring is 5 nm-1 um; and the thickness of the nano-ring is 1 nm-500 nm. The sensor utilizes the ferromagnetic nano-ring under the action of the outside magnetic field, and the resistance the ferromagnetic nano-ring changes greatly, so as to detect the change of the outside magnetic field; the manufacture is very simple; the output signal is large; and the response is fast. The sensor is especially applicable to the detection of the threshold value of the magnetic field change; besides, the magnetic field sensitivity of the magnetic field transformation point can reach 2-10 percent/Oe or is higher than 2-10 percent/Oe, and the sensor also can serve as a magnetic cell.