Near-infrared (NIR) II luminescence between 1000 and 1700 nm has attracted reviving interest for biosensing due to its unique advantages such as deep-tissue penetration and high spatial resolution. Traditional NIR-II probes such as organic fluorophores usually suffer from poor photostability and potential long-term toxicity. Herein, we report the controlled synthesis of monodisperse NaCeF:Er/Yb nanocrystals (NCs) that exhibit intense NIR-II emission upon excitation at 980 nm. Ce in the host lattice was found to enhance the luminescence of Er at 1530 nm with a maximum NIR-II quantum yield of 32.8%, which is the highest among Er-activated nanoprobes. Particularly, by utilizing the intense NIR-II emission of NaCeF:Er/Yb NCs, we demonstrated their application as sensitive homogeneous bioprobes to detect uric acid with the limit of detection down to 25.6 nM. Furthermore, the probe was detectable in tissues at depths of up to 10 mm, which enabled imaging of mouse organs and hindlimbs with high resolution, thus revealing the great potential of these NaCeF:Er/Yb nanoprobes in deep-tissue diagnosis.