In the present paper, the characteristics of low frequency stable damping is studied in a multi-phase eutectiod Al-Zn alloy. The experimental results show that, within a low temperature and low strain amplititude range, the damping evidently manifests itself by linear viscous characteristics, namely, it obeys a law Q=(B/f) exp(-nH/kT ), where H is the real process activation energy, B and n(=0.21)are two experimental parameters and k is the Boltzmannc constant. A value of H=0.74eV is obtained, which is closely related to the viscosity during interface motion. Accordingly, a linear viscous interface motion model is proposed. Using this mode we can explicitly explain the experimental results.