Antigorite is the main carrier of water in Earth’s subduction zones. The dehydration processes of antigorite were investigated by carrying out in situ phase transition experiments using a dynamic diamond anvil cell, with a time-resolved Raman scattering system, at 0.3–10 GPa and 396–1100 K. Three typical phase transformation reactions occurred within the P–T range of this study, corresponding to three reaction products. At low pressures (8.6 GPa), the products of the dehydration phase transition of antigorite are consistently clinoenstatite and phase A. Compared with the previous studies carried out by large-volume presses (such as a multi anvil press and a piston-cylinder press), the reaction to produce phase A occurs at higher P–T conditions, and the stable temperature region for talc as a dehydration product is narrower. Moreover, large quantities of pores with 5–10 μm in diameter formed in dehydration products, supporting the hypothesis that intermediate-depth earthquakes may result from dehydration embrittlement. The precise phase boundary determined by this in situ study provides a better understanding of the dehydration phase transition behavior and geological phenomena exhibited by antigorite under different pressure and temperature conditions.