The development of effective approaches for detecting toxic hydrazine (N2H4) is of great importance for public health. In this work, a near-infrared (NIR) fluorescent probe (QLN-Hy) for trace hydrazine detection with a large Stokes shift was constructed. Hydrazine-activatable QLN-Hy displayed striking characteristics of high sensitivity (detection limit: 0.25 μM), high selectivity, as well as notable fluorescence enhancement at the NIR emission (692 nm) and large Stokes shift (104 nm). In particular, the color of QLN-Hy changed from purple to light blue, enabling the direct visualization of hydrazine in vapor and aqueous phases through the naked eye readily. Meanwhile, the practical feasibility of QLN-Hy was successfully evaluated by quantitatively detecting trace hydrazine in water samples (lake water, river water, rain water, tap water, drinking water) and mice samples (urine, serum). Importantly, probe QLN-Hy can be conveniently applied to monitor and visualizing hydrazine in cells and in vivo, clearly demonstrating its invaluable potential biological applications.