Abstract: From the basic define of the inductance, a new formula to calculate dynamic inductance is deduced, which can figure out the nonlinear inductance of winding with magnetic cores easily by numerical calculation. The dynamic inductances of a pure winding in 50 Hz alternating current, a 5 rounds winding with a ferrite core in 50 Hz alternating current and a coaxial cavity with 17 ferrite cores in double pulses exciting current are calculated to prove the accuracy of the formula. The magnetic core’s permeability can be figure out from the inductance, which make it easily to confirm the applicability of the magnetic core.