Near-infrared-II (NIR-II) fluorescence visualization is an in vivo optical imaging technique with large imaging depth and high resolution. Compared with visible (360?760 nm) and near-infrared-I (760?900 nm) windows, the light absorption of bio-tissues in the second near-infrared window (900?1880 nm) improves significantly, suppressing the scattering background in two-dimensional array detection, thus remarkably improving imaging quality. Currently, scientists have developed various luminescent probes, imaging systems, and theranostics pathways corresponding to the NIR-II window. The experimental animals cover from rodents to non-human primates. The clinical surgery of human liver tumors guided by NIR-II fluorescence imaging has also been initially evaluated. With light absorption and photon scattering in the visible and near-infrared regions in biological tissues, this study expounds on the mechanism of imaging quality improvement on NIR-II fluorescence imaging. It briefly introduces several typical NIR-II luminophores and reviews the developed multimodal NIR-II optical imaging methods. The multi-functional theranostics platform guided by precious NIR-II optical imaging is also discussed. These aim to provide references for further development and accelerate clinical translation.