The isothermal forging of a cast Mg-5.89 wt.% Zn-0.79 wt.% Y-0.32 wt.% Zr magnesium alloy was carried out to form the tank cover. The forming process of isothermal forming is studied by finite element model simulation and experiment. Based on the optimum isothermal forging process, the isothermal forging experiments of the cast Mg–Zn–Y–Zr magnesium alloy tank cover were successfully conducted. The results show that the experimental results such as forming load are in good agreement with the calculated ones. The tank cover forging with complete die fill and without fold defect can be obtained by utilizing the optimum isothermal forging process. The mechanical properties of the tank cover show a significant improvement over those of original materials, and the tank cover exhibits an ultimate tensile strength of 302 MPa, a yield strength of 209 MPa, and an elongation of 13%. © 2017, Springer-Verlag London Ltd.