Sensitive and selective detection of hypochlorite is in great demand for food safety, especially in fresh cold chain products. However, the detection limit of traditional visible emission-based strategies cannot satisfy the requirement of ultrasensitive analysis in practical applications. In this work, we explored a novel luminescent nanoprobe in the near-infrared-II (NIR-II) window to greatly improve the hypochlorite detection limit for analysis of real milk samples, which was based on the fluorescence resonance energy-transfer process between the hypochlorite-responsive dye (FD1080) and the lanthanide-doped downconverted nanoparticles. Specifically, the NIR-II luminescence from Yb ions was first suppressed by FD1080 due to the energy-transfer mechanism. In the presence of hypochlorite, FD1080 was bleached to recover the luminescence. As a proof-of-concept, the optimal nanoprobe exhibited a linear luminescence recovery in the range of 0.1-1 nM with the detection limit of 0.0295 nM for hypochlorite. Real milk sample detection experiments showed that the probe had good accuracy and precision.