In order to control the mechanical properties of heavy forgings, it is important to control the grain size of material. The characteristics of the austenite grain size of A5083 steel under different deformation conditions, at different temperature and holding time are studied by high temperature compression experiments and heating & holding experiments. The compression experiments are carried out at 1 050 ℃ and 1 200 ℃ to a strain of 10%, 20%, 30%, 50%, at the same strain rate of 1×10-3s-1. The compression experiments results show that the grain size grade (GB63941986) increases by one level when the deformation is less than 30%, and increases by two levels when the deformation is more than 50% at the temperature of 1 050 ℃. The grain size grade increases by one level when the deformation is less than 50% at 1 200 ℃. The heating & holding experiments results showe that the grain size will not change significantly by two hours’ holding at 1 050 ℃, and changed little by one hour’s holding at 1 200 ℃.