With high strength-to-weight ratio and excellent technological properties, Magnesium Alloy plays a more and more important role in automotive industry. But the inferior high temperature creep resistance limited its application to power components such as engine and transmission cases. In order to investigate the cyclic creep behavior of Magnesium Alloy at high temperature, creep tests of plate specimens AM50 were conducted. Through the analysis about the microstructure and defects of AM50, an isotropic and scalar damage parameter was introduced into an existing creep constitutive model. Furthermore, a Fortran code based on numerical algorithm was developed to simulate plasticity, cyclic creep, and recovery phenomenon observed. Comparisons between calculated results and experimental data show good agreement.