Near-infrared II (NIR-II) fluorescence imaging technology has emerged as a research hotspot in the field of biomedical imaging due to its advantages including deep tissue penetration, high resolution, and low background interference. However, traditional probes still face challenges in terms of penetration depth, resolution, signal-to-noise ratio, real-time performance, and safety. This study comprehensively reviews the research progress of inorganic and organic NIR-II fluorescent probes, including the design strategies, performance optimization, and applications in in vivo imaging, tumor diagnosis and therapy, and vascular visualization of single-walled carbon nanotubes (SWCNTs), quantum dots (QDs), rare-earth nanoparticles (RENPs), gold nanoclusters (AuNCs), as well as organic small-molecule dyes. Through molecular engineering, surface modification, and material innovation, researchers have successfully enhanced the quantum yields, biocompatibility, and targeting capabilities of these probes, and developed high-performance probes for the NIR-II window. This study aims to provide guidance for the further development of NIR-II fluorescent probe technology and its clinical application, as well as to explore its future directions.