In order to understand the law of deformation behavior in the process of cyclic closed-die forging (CCDF), to improve the preparation process and to lay the foundation for further industrial application, the finite element analysis was performed on CCDF by numerical simulation software Deform-3D. The features of temperature field, strain field, velocity field and stress field of the sample in die forging were discussed. The results show that the upsetting deformation of the upper end of billet is earlier than the lower end when filling the cavity. Most parts of the internal sample bear compressive stress in three-dimensional directions, and the shear plastic deformation always exists because of different values and directions of flowing speed in forging. Therefore, the strain distribution is very inhomogeneous on the whole sample after an initial pass, and both strain uniformity and accumulated strain increase as sequential passes are carried on. The temperature field simulation shows that the temperature rising in the edge and surface are very apparent, while it is lower in most areas of the internal part. The experimental investigation of processing cast magnesium alloy AZ31 at 400 ℃ shows that the average grain size is refined about from 178 μm to 19 μm after four passes, and the distribution of grain is homogeneous.