Abstract: Starting from the principle which inductors follow for energy storage，taking a one-order RL circuit driven by positive-negative pulse source as object and using math deduction，we get the relation between frequency and inductor's reactance in the steady state. The relation is also verified by using simulating experiment. Based on the same physical senses，i.e.，energy storage and charging-discharging，the explanation for the performance and properties of inductor in transient of dynamic circuit and in sinusoidal steady state circuit is given. The results are helpful to understand and master the concept of reaction of energy storage elements in the sinusoidal steady state.

Key words: reactance, cycle, frequency, charge and discharge, energy storage element