We have developed a new lysosome-targeting ratiometric fluorescent probe (FL-HA) for specific determination of HOCl in living cells. The developed probe exhibits accurate lysosome-targeting ability, good signal resolution, and high selectivity. Moreover, fluorescence imaging shows that FL-HA could be used as a probe for ratiometric visualization of exogenous and endogenous HOCl in lysosomes with a high colocalization coefficient compared with a commercial lysosomal location dye.