MgAl-hydrotalcite compounds (MgAl-HT) with different n(Mg)/n(Al) ratios were synthesized by coprecipitation method. XRD and FT-IR were performed to characterize the structure of MgAl-HT. TG-DTG technology was adopted to study the thermal decomposition behavior. It was discovered that the thermal decomposition proceeded in two stages when the ratios of n(Mg) toward n(Al) being to 2~4. The kinetic parameters of the thermal decomposition procedure were also calculated by Ozawa method, which demonstrated that the value of active energy showed dynamic distribution. At each step of the decomposition behavior, the value of active energy decreased at first and then rose. The value of active energy for the deformation of the water in the interlayer was located within 45~90 kJ•mol-1 and within 150~230 kJ•mol-1 for the deformation of structure hydroxyl. Those characteristics were discussed and explained according to relative theory.