The principle of external sphere grinding of hemispherical Resonator based on generation method is discussed. The theoretical calculation model of curvature radius was established. The dominant errors affecting the machining precision of external sphere are shape error of diamond tool, positioning accuracy of CNC machine axis and runout of CNC machine shaft. The shape error of diamond tool includes the diameter error of diamond tool and radius error of diamond tool edge. The positioning accuracy of CNC machine is determined by the positioning precision of X-axis, Y-axis and B-axis.The runout of CNC machine shaft includes axial and radial runout errors. After optimizing the machining process parameters, the roundness of the hemispherical resonator is less than 0.25μm, the concentricity of the hemispherical resonator is less than 0.4μm, the surface roughness is less than 0.02μm and the Q value is greater than 2.6×107.