The phase diagram of the SnO2-SiO2 system was investigated using sealed Pt crucibles. A eutectic reaction of L → SiO2 (s) + SnO2 (s) was experimentally determined at about 1670 °C and 1.15 mol % SiO2. No mutual solid solution between SiO2 and SnO2 was found. To explain this eutectic, a large liquid-liquid miscibility gap with a monotectic at about 1821 °C must exist between SnO2 and SiO2. Based on the present phase diagram experimental data of the SnO2-SiO2 system and the available thermodynamic and phase diagram data for the binary SnO-SiO2, SnO-SnO2, and ternary SnO-SnO2-SiO2 systems found in the literature, the thermodynamic optimization of the entire SnO-SnO2-SiO2 system was carried out. The new phase diagram of the entire SnO-SnO2-SiO2 system at 1 atm total pressure was constructed from the optimized Gibbs energy functions of all phases. The optimized database was also applied to simulation of the evaporation and condensation phenomena in float glass making process.