Abstract: The reaction mechanism of dehydration process of Guyang limonite was studied by TG-DTA and X-ray diffraction. The thermal analysis data were processed and analyzed by the liner correlation calculation with different dynamic mechanism models. The dehydration reaction mechanism was determined, meanwhile, the kinetic parameters of dehydration reaction were calculated by Coat-Redfern integral method. The results showed that the dehydration reaction process of Guyang limonite is divided into two stages. The first stage is at 140~320℃ with a weight loss rate of 4.78% and a reaction activation energy of 28.54 kJ/mol. The reaction mechanism of the first stage is in accordance with Avrami-Erofeev equation, which belongs to the stoichiometric and subsequent growth of the primary chemical reaction. While the second stage is at 320~465℃ with a weight loss rate of 3.84% and a reaction activation energy of 61.92 kJ/mol. The reaction mechanism of the second stage agrees with the secondary reaction function, and it is controlled by chemical reactions. XRD results showed that the phase transition of limonite occurs at 200℃ with increasing temperature, during which the diffraction peaks of goethite are weakening and disappearing, but the diffraction peaks of hematite gradually increase. As a result, the mineral phases become pure.