The method of crucible rotating oscillation damping is employed to measure the kinematic viscosity of Mg-9Al melt, and the curve of viscosity ν versus temperature T from 890 K to 1190 K is obtained. It is found that there is an abnormal change for the viscosity in the first heating process, i.e., when the temperature is increased to 1000—1075 K, the viscosity varies from increase to decrease. However, in the subsequent cooling process and the second heating process, the viscosity increases (decreases) monotonically according to an exponential law with temperature, which accords with the Arrhenius equation. Based on the residual bond model and the calculation model for evolution behavior of "average atomic cluster", the correlation between viscosity and microstructure of Mg-9Al melt is discussed. The results show that the breakage of Al-Al(B) bonds in basic unit of β phase-like residual bond structures causes an abnormal change of viscosity in the first heating process; in the subsequent cooling process and the second heating process, the melt reaches a new dynamic equilibrium state, and Al atoms are uniformly distributed in the melt. At this time, the size of Mg-Al average atomic cluster dS and the number of short-range order atoms NS inside them increase (decrease) monotonically with temperature, and the relationship between viscosity ν and size of average atomic clusters dS is expressed as a linear function, i.e., ν = ν0 + K·dS, which presents a new way for revealing micro-structure change of alloy melt and further understanding the change characteristic of viscosity.