The damping mechanism of a magnesium alloy was studied in the process of its vibration deformation.Tt was discovered that before the macroscopic yiele stage,with the increase of the strain ampoitude,the damping capacity can be divided into four stages:resonated damping during dislocation elastic edformation,static hysteresis damping during dislocation break loose pinning (slip breeding),dislocation line entwisted and the slip band generated.According to the initial strain amplitude of each damping stagein deducing process,the damping value in large stain amplitude shoule be tested in experiments.The testing method and device were designed to acquire the spectrum of damping strain of each stage through one excitation.Using the transient excitation and making the rectangular magnesium alloy beam vibrate with large strain amplitude; the data of each damping capacity stage was acquired by testing the vibration signal in the free damping process.The method was proved to be feasible and the conclusion of deducing was proved to be right according to the damping test of magnesium alloy AZ61D.