Al2O3 ceramic was added to a CaO-B2O3-Al2O3-SiO2 glass system to improve mechanical properties and thermal cycle stability in solid oxide fuel cell composite seals. The performance of glass-based seals with different Al2O3 content was evaluated, including the shear strength and leakage rate of the composite seals. The results showed that HA80 composite seal, with 20 wt% Al2O3, had excellent joint strength and gas tightness. HA80 had the highest shear strength among specimens, reaching 3.83 MPa after heat treatment at 750 degrees C. Gas tightness was greatly improved by using HA80 composite seals, with leakage rates from 0.0007 sccm/cm to 0.0011 sccm/cm corresponding to gas input pressures varying from 13.6 to 68 kPa. The leakage rates remained stable about 0.0022 sccm/cm at the highest gas input pressure (68 kPa) during twenty thermal cycles because of improved interfacial bonding between the composite seals and adjacent components. The 5-cell stack was assembled and achieved an open circuit voltage of 5.78 V and maximum power output of 354 W at 750 degrees C, demonstrating that HA80 composite seal was suitable for SOFC application.