In the process of metal plastic forming severe plastic deformations occur to the workpiece. As a result the constitutive relations between material and geometry are nonlinear. One of the most common tools to solve such problems is the finite element method. In thixoforming processes of semi-solid metal many factors affect thixoforming fluidity. Therefore it is important to simplify the problem and predict the metal flow properties. For that reason the thixoforming process of semi-solid AZ91D magnesium alloy was modeled with the theory of rigid viscoplastic finite element method using the commercial finite element software DEFORM-3DTM. The fluid and effective stress-strain fields in the thixoforming process were obtained and the relationships among stress, strain rate and temperature were analyzed.