Hydrazine (N2H4) has high toxic impact and is widely used as raw material. As a result, it poses serious health risks and the following environmental pollution. Therefore, hydrazine's detecting techniques remains constantly under investigation. Herein, we report a ratiometric D-π-A fluorescent probe displaying NIR emission based on the ICT mechanism to detect N2H4. Other tested analytes have no observable interferences on the sensitivity and selectivity. The probe has sensitivity for N2H4 down to 1.29 μM with quick detection time (about 2.0 min). Moreover, the probe demonstrates solvatochromism with obvious emission varying from dull red to green, which implied high sensitivity to medium polarity. A portable smartphone platform for point-of-care detection of N2H4 in water samples is constructed. As a proof of real samples, the probe is successfully applied to sense N2H4 in water samples, to bioimage N2H4 in living cells and in plant cells with low cytotoxicity, suggesting significant potential application. This strategy opens up the preparation of new ratiometric fluorescence systems for bioimaging and environmental monitoring applications.