Objective To study the influence of hydrothermal time on hydroxyapatite ( HA) synthesis of microarc oxidized TC4 alloy. Methods The microarc oxidation ( MAO) coatings on TC4 alloy were treated by different hydrothermal synthesis time. The morphology, composition and phase structure of different coatings were analyzed by SEM, EDS and XRD. Besides, the changes of morphology and phase structure were studied when different MAO coatings were soaked in SBF for 5 weeks. Results Hydrothermal synthesis increased the molar ratio of Ca and P in MAO coating, and changed the amorphous calcium phosphate compounds into HA crystals. The diffraction peak of HA was increased and enhanced continuously with the increasing hydrothermal synthesis time. Only a little calcium phosphate was produced when MAO coating was soaked in SBF for 5 weeks, whereas the HA crystals of MAO coating treated by hydrothermal synthesis were almost completely converted to calcium phosphate after 5 weeks soaking in SBF.Conclusion Hydrothermal synthesis was helpful to form HA crystals on microarc oxidized titanium. HA crystals increased when hydrothermal synthesis time was extended from 0 to 8 h, and then more calcium phosphate was produced on the surface of MAO coating after soaking in SBF, which accordingly had better compatibility with human body.