Putting a soft ceramic in a sandwich of hard ceramics will produce composites combining the merits of both soft ceramics and hard ceramics. To strengthen soft ceramics, two analytical relationships among the bending strength, the residual stresses, and the ratio of the coating thickness to the substrate thickness, R, in sandwich beam samples with strong interfaces were established based on the three-point bending model. When the temperature drop and the material properties of the coating and substrate are fixed, the strength enhancement due to the residual stress can be predicted. Furthermore, an optimum ratio R0 was derived using a stress equilibrium principle, which makes the designed component having the highest strength. These predictions were confirmed by using a bending test on the hard–soft–hard sandwich samples of Al2O3/Ti3SiC2/Al2O3. The measured maximum strength was 14.5% higher than that of Ti3SiC2 when R was 0.10, which was close to the calculated optimum ratio R0 (0.087).