Cu2O-Al2O3-SiO2 ternary system of given compositions can be used as a kind of thermal shock resistance sealing materials for electronic devices. So far, little systematic work was reported on the phase behavior and phase equilibrium at a high temperature, except the isothermal section of a CuO-Al2O3–SiO2 ternary system at 950 and 1150°C. We have investigated the thermodynamic behavior and the microstructure of the compositions in the miscibility gap of the Cu2O-Al2O3-SiO2 pseudo-ternary system on the basis of the reported isothermal section of this system at 1150°C based on the analyzing of the differential scanning calorimeter and the observing of the scanning electronic microscope. The eutectic point of the Cu2O-Al2O3-SiO2 pseudo-ternary system was determined by the thermal analysis. Then, the changing rule of the microstructures with different compositions was investigated. The results indicated that large differences existed among the different compositions in the miscibility gap of Cu2O-Al2O3-SiO2 ternary system.