The traditional operation criterion of distance protection is to compare some measured impedances with a setting impedance. Due to the effect of decaying DC component, the measured impedances are generally imprecise in both amplitude and phase angle, which may result in incorrect operation of the protection. Based on the unmodified discrete Fourier transform, this paper deduces the phase angle error and relative amplitude error respectively caused by the decaying DC component in ground distance protection and interphase distance protection. On the basis, a novel calculating algorithm for measuring impedance without filtering decaying DC component is proposed. By recombining the amplitude and phase angle to generate a new impedance, it efficiently eliminates the effect of decaying DC component. As a result, the new generated impedance can be accurate both in amplitude and phase angle. PSCAD simulation verifies the validity of the proposed algorithm.