［Mg(H₂O)₆］(NTO)₂·2H₂O was prepared by adding magnesium carbonate hydroxide to the aqueous solution of 3-nitro-1,2,4-triazol-5-one (NTO). Its thermal decomposition mechanism was studied by DSC,TG/DTG and IR. The quantum chemical calculation on the title complex as a structure unit with the experimental geometry as starting values was carried out at B3LYP level with 6-31G basis set. The results show that the bonds between the coordinate waters and the Mg atom have certain extent covalent character. The net charges on nitrogen atoms of the NTO ring appear to be negative while the nitrogen atom on the nitro group (—NO₂) appears to be positive which indicates —NO₂ will lost first when the complex is heated to some uniform temperature and this result is in agreement with that of the thermal decomposition experiment.