Ultra-high quality factor whispering gallery mode crystal microresonators have promising applications in nonlinear optics, coherent optical communication, and microwave photonics. Improving the surface roughness to reduce the scattering loss is an effective way to improve the quality factor of magnesium fluoride (MaF₂) crystal microdisk resonators. In this paper, we fabricated ultra-smooth MaF₂ crystal microdisk resonators with sub-nanometer surface roughness based on ultra-precision machining method, and the quality factor Q of MaF₂ crystal microdisk resonators in 1550 nm band is up to 1.2×10⁹ by using the tapered fiber coupling combined with cavity ringdown method, which helps to promote the research development in the field of ultra-high quality whispering gallery mode crystal microresonators in China.