Abstract: The saturable inductor used in high voltage direct current（HVDC） converter valve’s（It is often called saturable reactor in electric power system.） is a fast saturable inductor. It needs to switch frequently between unsaturated state and saturated state when it works. In order to study the core loss of inductor under such special condition, the circuit for measuring the core loss of fast saturable inductor was designed by imitating its working conditions, extracting fundamental data related to core loss. Based on volt-second balance principle, the excitation is equivalent to extremely small pulses. Since the effective excitation frequency of the fast saturable inductor is greater than working frequency, the modified Steinmetz equation（MSE） formula is corrected by using the effective excitation frequency instead of working frequency and modifying its exponent. The core loss model of fast saturable inductor under extremely small pulse voltage is obtained. Finally, the experimental results verified the higher accuracy of proposed model than MSE.