The synthesis of high-quality single-crystal covalent organic frameworks (COFs) presents significant challenges, particularly in achieving precise control over their morphologies. Herein, we present a straightforward strategy to fine-tune the morphology of COF single crystals. Using rigid triptycene derivatives as the core building blocks and varying the amounts of aniline modulators, we successfully synthesized a series of high-quality COF single crystals with different aspect ratios and well-defined facets, JUC-663-X (X = 30 to 135, the equivalent of aniline). Their structures were characterized using PXRD, TEM, and N2 adsorption analyses to confirm the structural consistency. The study of the growth mechanism and DFT calculations elucidated the crucial role of aniline as a modulator in facilitating anisotropic competitive binding throughout the crystal growth process. Furthermore, our findings demonstrate that the aspect ratio of these single crystals significantly influences the adsorption properties of Rh B. This research not only paves new paths in the synthesis and morphological control of COF single-crystal materials but also provides profound insights into the relationship between COF morphology and functional performance.