The reaction between Si3N4 and AlF3 had been investigated in CO atmosphere at high temperature. Experimental results are shown that the reaction between Si3N4 and AlF3 is accelerated with the rise of temperature. At the temperature of 950 (aluminum electrolytic operation temperature is 935±15), there is obvious reaction between Si3N4 and AlF3; under this experiment condition, Si3N4 converted into Si2N2O partly, Si2N2O can also react with AlF3. The reaction products of Si3N4 with AlF3 are SiF4 (g) and AlN, while the reaction products of Si2N2O with AlF3 are SiF4 (g) and Al8O3N6. So, during the usage of Si3N4-bonded SiC sidewall brick in aluminum electrolysis cells, the content of Si3N4 in Si3N4-SiC block are reduced which can bring down the corrosion of aluminum electrolyte to the sidewall materials.