The surface resistance reduction can further suppress the surface power loss of a superconducting radio-frequency cavity. Since the surface resistance of a superconducting radiofrequency cavity is mainly originated from the magnetic flux trapping, and thus the corresponding magnetic field strength could be measured to reflect the residual resistance. A fluxgate magnetometer is always employed to measure the ambient surface magnetic field of a superconducting radio-frequency cavity. However, this kind of equipment is relatively larger than the cavity and always need expensive cost. In this paper, we developed a magneto-inductive (MI) magnetic sensor, which is smaller, lighter weight, and lower cost than the fluxgate magnetometer. The specifications such as the noise floor, resolution, etc., are measured. In addition, a comparative observation of the magnetic field between the proposed MI sensor and a highly precise is conducted. The experimental results identify the capability of the proposed MI sensor in weak magnetic detection.