Abstract: In order to evaluate the impulse magnetic field distribution in buildings struck by lightning or nearby lightning strokes, a set of measuring system composed of selfmade magnetic detecting coil and fiber transmitting system was developed. The sensitivity coefficient of the fiber transmitting system was calibrated by sinewave generator. The output of an 8/20 μs current generator is connected with the single turn circular coil whose diameter is 0.3 m to compose the magnetic field generator. The detecting coil is fixed in the center of the single turn coil for calibration. The results show that the developed system can measure the impulse magnetic field effectively and the B/U calibrated coefficient error of the system is less than 3%.