Abstract: To obtain the damping property of alumium alloy plate－damping ratio ξ，this paper introduced the method of cantilever beam damped free vibration for two kinds of aluminum alloy plate ( normal Al-alloy and high damping Al-alloy) ，ξ value was obtained easily and precisely by this method．The theory fundation，implementation condition，operation process and precision verification by simulation of the method was discussed in detail．Processing and analysis of testing data show that ξ value of high damping Al-alloy is 1.8 times higher than normal Al-alloy，and the value is not constant．The results provide reference for future design of structural vibration attenuation and optimization．