The whispering gallery mode (WGM) microcavity has ultra-high Q factor and an extremely small pattern volume, and has a broad application prospect in the fields of microwave photonic systems, nonlinear optics and quantum optics. By analyzing the loss factors of millimeter-scale magnesium fluoride (MgF₂) crystal WGM microcavity, it was confirmed that the main indexes affecting the quality factor of WGM microcavity were material grade and surface roughness. The structure of WGM microcavity was designed with millimeter-scale MgF₂ crystal, and the DUV-level MgF₂ crystal was used. Assuming the surface roughness of WGM microcavity was less than 0.7 nm, the theoretical calculation value of limit loss was 4.781×10^?11 and the corresponding limit Q value was 2.09×10¹⁰. Through rough machining, precision turning and precision polishing of WGM microcavity of MgF₂ crystal, the microcavity manufacturing with high quality factor was realized. The test results show that the surface roughness of WGM microcavity is 0.669 nm (Ra value), the shape error is 6.767 nm (PV value), and the quality factor is 2.054×10⁹@1 550 nm.