In summary, we designed and synthesized NIR-II AIEgens with ultrahigh QYs based on the donor engineering. By thorough experimental and theoretical calculations, we revealed the two key factors for obtaining dye molecules with ultrahigh QYs by constructing D-A structures using TPE as a donor. The optimized D-A interaction could mediate the strong TICT effect as the TICT effect would severely quench the luminescence in the aqueous environment. Thus, the mitigation of TICT effect would effectively raise the QY. Because TPA-BBT and TPEO-BBT showed stronger D-A interaction than TPE-BBT, TPE-BBT thus demonstrated the highest QYs in all the states. On the ohther hand, the results of single-crystal analysis, MD simulations, and reorganization energy calculations proved that TPE-BBT demonstrated a stronger RIM effect in aggregates because of the stronger intermolecular interactions.