A calculation on magnetic instability of superconductor has been made on the basis of a thermal differential equation. Taking magnetic and thermal quasi-equilibrium conditions in superconductor in an increasing magnetic field, the author derived the upper limit of the equilibrium, B_fj which shows the same value of stabilization limit already derived by Swartz and Bean with a method of infinite incremental permeability.
By means of a numerical method on the thermal differential equation, the increase of temperature of superconductor was calculated in function of the increase of an applied field, B_a. This calculation gave B_fj with the parameters of dB_a/dt, cooling conditions and copper ratio in superconductor. The calculated value of B_fj for no cooling condition agrees with experimentally measured flux jump level.