Rigid-plastic finite element method considering thermo-mechanical coupling was used to investigate the thermal forging process of a cubical-shaped 7075 aluminum alloy piece, and the Yada model was adopted to calculate the grain refinement in the process. The simulation results show that the distribution of stress, strain, strain rate and temperature of the forging piece are not uniform, and according to the distribution of these physical quantities, the forging piece can be divided into four characteristic zones: the center ellipsoidal zone, the contact edge zone, the vertical edge zone and conical contact zone, where the first two zones are easy to deform, and the remained two zones are difficult to deform. The grain refinement begins from the center ellipsoidal zone, and then gradually expands to the vertical edge zone and conical contact zone. The comparison analysis of different forging parameters shows that temperature and compression strain rate is the major forging parameters controlling grain size. The grains of 7075 aluminum alloy can be further refined with the increase of compression strain rate, but high compression strain rate will bring the excessive heat that will make grains to grow. Then a set of suggested forging parameters are given as follows, the initial temperature should be 350℃~400℃, the compression strain rate should be 70 s-1~100 s-1, and the compression ratio should be 20%~25%. And finally, the limitation and application region of Yada model are also discussed.