LiErF₄was commonly used as a dipolar-coupled antiferromagnet, and was rarely considered as a luminescent material. Herein, we achieved the strong red upconversion emission of LiErF₄simply by an inert shell coating, i.e., LiErF₄@LiYF_4.Owing to the unique and intrinsic ladder-like energy levels of Er³⁺ions, this LiErF₄core-shell nanostructures present red emission (similar to 650 nm) under multi-band excitation in the near-infrared (NIR) region (similar to 808, similar to 980, and similar to 1,530 nm). A brighter and monochromic red emission can be further obtained via doping 0.5% Tm³⁺into the LiErF₄core, i.e., LiErF₄: 0.5% Tm³⁺@ LiErF₄. The enriched Er³⁺ions and strong monochromic red emission natures make LiErF₄: 0.5% Tm3+@ LiErF₄nanocrystals very sensitive for trace water probing in organic solvents with detection limit of 30 ppm in acetonitrile, 50 ppm in dimethyl sulfoxide (DMSO), and 58 ppm inN,N-dimethylformamide (DMF) under excitation of 808 nm. Due to their superior chemical and physical stability, these nanoprobes exhibit excellent antijamming ability and recyclability, offering them suitable for real-time and long-term water monitoring.