Abstract: For the high altitude electromagnetic pulse testing research purpose, the measuring principle and frequency characteristic of electrically small loops are analyzed in this paper. A numerical model of electrically small loop is founded, and the pulse receiving characteristics are obtained by simulation. Then, the B-dot sensor is designed, and a pulsed magnetic field measuring system is constructed. The results show that the sensor's upper cut-off frequency exceeds 720 MHz when the ring radius is less than 20 mm, and the system calibrated risetime is less than 1 ns. The measuring system is used to measure the magnetic field generated by a HEMP simulator. It shows that the system can measure pulsed magnetic field with fast risetime, and it can also be applied to measure other pulsed magnetic field with high magnitude.