Increasing attention has been paid in recent years to alpha-tricalcium phosphate (alpha-TCP), which is used as the main constituent of calcium phosphate bioactive bone cements and biphasic calcium phosphate ceramics. Its hydrolysis and conversion into apatite phase may play an important role in new bone formation in vivo. In this experiment, alpha-TCP powder was made and immersed in deionized water. The morphology change and phase composition were analysed before and after immersion. Based on the results of hydrolysis, a formula for the converted apatite-TCP phase containing lattice water was put forward and a partial structural model along the hydroxyl column was proposed.