Compared to the visible and near-infrared-I （NIR-I） spectral regions， the near-infrared-II （NIR-II， 1000–1700 nm） window has emerged as a promising frontier in bioimaging due to its greater tissue penetration depth and reduced spontaneous background fluorescence. In recent years， significant progress has been made in applying Er³⁺-rich systems （NaErF₄） as NIR-II fluorescent probes in biomedical photonics. These studies not only focus on enhancing imaging signal-to-noise ratios and detection sensitivity but also aim to achieve precision imaging-guided therapy for tumor regions. This review systematically summarizes the design strategies， luminescence mechanisms， and recent advances in the biological applications of NaErF₄-based NIR-II luminescent nanomaterials. Additionally， the challenges associated with their future translation are discussed.

NaErF₄;NIR-II;downshifting emission;fluorescent imaging;imaging-guided therapy