A water-based tape-casting slurry is reported to prepare the ceria and scandia co-doped zirconia (ScSZ) electrolyte films. The slurry is characterized and optimized through Zeta potential and rheological property measurements. Smooth and flat ScSZ electrolyte films are obtained by improving the sintering process. The microstructure, electrical performance, and mechanical property of ScSZ with adding different contents of Al2O3 are also investigated. The results show that a proper amount of Al2O3 has a beneficial effect on the densification of ScSZ. A significant decrease in the grain boundary resistance of ScSZ is observed by adding 0.5 wt% Al2O3. The bending strength of the sample with 0.5 wt% Al2O3 (ScSZ-0.5A) is about 400 MPa, which is 20% higher than pure ScSZ. The ScSZ-0.5A electrolyte film fabricated by the water-based tape-casting method shows appropriate electrical conductivity and high mechanical strength, which is promising for practical application in solid oxide fuel cells (SOFCs).