The hydrotalcite with the properties of flame-retardant, eliminating smoke, filling and thermostability is a new kind of inorganic flame retardant. In the work, the MgAl hydrotalcite as flame retardant with Mg/Al molar ratio of 4 (MgAl-LDH) was prepared by using urea as the precipitating agent. The thermolysis behavior of the MgAl-LDH flame retardant was investigated by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR) and thermogravimetry-differential scanning calorimetry (TG-DSC) as well as self deconvolution and curve-fitting analyses. Thermal phase transition of the MgAl-LDH was clarified, especially the characteristics of the hydroxyl groups (—OH) in the brucite-like layers and the changes in coordinate of the carbonate (CO2-3) from the interlayers. Based on thermodynamic data, thermal decomposition process was discussed. By XRD analysis, it was found that the phase change took place when the decomposition temperature increased. The MgAl-LDH was decarbonated basically to MgAl mixed metal oxides (Mg—Al—O) at 500 ℃, and impurity MgAl2O4 phase formed at 600 ℃. According to the analyses of FT-IR, TG-DSC and curve-fitting technique, the hydroxyl groups (—OH) in the brucite-like layers possessed three the ligands such as ［Al—OH—Al］, ［Al—OH—Mg］ and ［Mg—OH—Mg］ modes. Dehydroxylation of the brucite-like layers based on the binding forces, where the ［Mg—OH—Mg］ among the three modes was the most difficult to be removed during the pyrolysis process. In the same way, the CO2-3 ligands also possessed three modes such as H2O-bridged CO2-3, monodentate and bidentate coordination modes. Based on the thermodynamic analysis, the thermodynamic properties of the hydrotalcite as flame retardant were evaluated, and the expressions of the Gibbs free energy (ΔrGθT), as a function of temperature, were derived for the Mg8Al2(OH)20CO3 crystal. Thermodynamic analysis showed that the removal of —OH from the brucite-like layers was spontaneous process, when the Gibbs free energy (ΔrGθT) was under zero at the temperature (T) above 228.65 ℃. The result and datum were close to the experimental result from the TG-DSC analyses, indicating that the relationship between the Gibbs free energy (ΔrGθT) and temperature (T) from thermodynamic analysis was reliable.