Two kinds of double salts were selected:kaolinite with high melting temperature and microcline rich in potassium with lower melting temperature.The reaction activity of kaolinite and microcline was calculated based on the density functional theory(DFT) of First Principle and the quantum chemistry ab initio calculation by using the generalized gradient approximation(GGA) and the PW 91 algorithm.Then the hinghest occupied molecular orbital, lowest unoccupied molecular orbital, density of states and Milliken population of kaolinite and microcline were analyzed.The results show that the value of energy level difference ΔE of kaolinite is quite large which reduces the chemical reaction activity and mullite forms after kaolinite loses H₂O molecular in high temperature.It is easy for kaolinite to react with the electron acceptor when it loses H₂O molecular, while it is difficult for kaolinite to react with the electron donor.Kaolinite becomes microcline when K⁺ as an electron accepter enters into the crystal lattice of kaolinite losing H₂O molecular which lowers the melting temperature of coal ash.