Isothermal precision forging is applied to manufacturing 7075 aluminum alloy disk workpiece with complex shape. By combining experiment with FEM (finite element method), the mechanism as well as the evolution with respect to the defects of the flow lines is studied in the isothermal precision forging of the workpiece. The results show that the flow lines of disk forging with complex shape are influenced mainly by the flow lines of the die forging billet. The radial flow lines distribute along the forging shape more easily than that the axial flow lines do. The fibre breaking is mainly caused by too much redundant metal flowing fast along the radial direction in the late stage of die pressing. In the end, some reasonable measures are put forward to optimize the flow lines.