The shear strength of composite secondary bonded single-lap joints was studied in this article. To optimize the adhesive thickness and ensure stable mechanical properties, an improved mold was applied. Based on this mold, a total of 15 specimens (180 samples) were examined and they were fabricated with various overlap lengths, curing pressures, adhesive thicknesses, ply angles, and surface treatment methods. The experimental results indicated that the improved mold not only significantly increased the uniformity of the adhesive thickness but also enhanced the shear strength of the joints and the shear strength was improved by approximately 13% compared to that of conventional methods. Moreover, the shear strength was decreased in specimens with increased overlap lengths and increased in samples with an increased curing pressure. Furthermore, the shear strength of the specimens was also affected by the adhesive thicknesses, ply angles, and surface treatment methods. The mechanisms can be ascribed to the effect of the fabrication method on the failure mode. A facile platform for optimizing these parameters is provided in this article. Based on this platform, the shear strength of the joints was enhanced to 33.5 MPa.