Fluorescence imaging in near-infrared IIb (NIR-IIb, 1500-1700 nm) spectrum holds a considerable promise for tissue imaging with deep penetration and high spatial resolution owing to the minimized autofluorescence and suppressed photon scattering. While few inorganic NIR-IIb fluorescent probes have been reported, their organic counterparts are still underdeveloped, possibly due to the lack of efficient materials with long emission wavelength. Herein, we propose a new molecular design philosophy to develop organic NIR-IIb fluorophores with high quantum yield (QY) by manipulation of the effects of twisted intramolecular charge transfer and aggregation-induced emission at the molecular and morphological levels. A pure organic fluorescent dye emitting up to 1600 nm with a QY of 14.2% in the NIR-II region (1000-1600 nm) is developed. For the first time, NIR-IIb fluorescence imaging of blood vessels and deeply-located intestinal tract of live mice based on organic dyes is achieved. The results show that organic fluorophore performs superb imaging ability in both superficial blood vessels and internal organs with high resolution and enhanced signal-to-background ratio in NIR-IIb region. We hope this groundbreakingly study will inspire further research on the evolution of pure organic NIR-IIb probes for in vivo imaging.