Fluorescence bioimaging through the second near-infrared window (NIR-II, 1000-1700 nm) has attracted much attention due to its deep penetration and high contrast. However, exploring new fluorescent materials, especially small molecular fluorophores with long wavelength and high brightness, is still quite challenging. By expanding π-conjugation and enhancing the intramolecular charge transfer effect, herein we report a series of new xanthene-based NIR-II dyes, named VIXs. Among these dyes, VIX-4 exhibits the best performance with fluorescence emission at 1210 nm and high brightness and has been used for dynamically imaging the blood flow of mice at 200 fps. By virtue of high spatiotemporal resolution of the dynamic imaging, we can distinguish directly the artery and vein through the blood flow direction and measure the blood flow volume by the videos. This study provides not only an effective tool for high spatial and temporal resolution bioimaging but also a new and promising conjugated skeleton for NIR-II dyes.