Excessive hypochlorite (ClO⁻) is easy to form residues in water, which will seriously endanger human health and environmental pollution. Therefore, it is essential to develop a sensitive fluorescent sensor to detect ClO⁻ in water. Herein, a simple and economical fluorescent probe for the detection of ClO⁻ was designed by highly exfoliated graphite-like carbon nitride (Ex_g-C₃N₄). The results showed that Ex_g-C₃N₄ had obvious fluorescence quenching effect on ClO⁻ with high selectivity and anti-interference ability, which was feasible for making probes for detecting ClO⁻ in water. Sensing experiments showed that the Ex_g-C₃N₄ probe had the detection limit of 5.56 nM while the detection range was 0–62 mM in water. Moreover, the fast response time of Ex_g-C₃N₄ was less than 30 s, illustrating the superior sensitivity. Besides, the fluorescence sensing experiment was carried out in various liquid conditions, which demonstrated that Ex_g-C₃N₄ probe had outstanding detecting application in natural environment. A portable fluorescent test strip for rapid detecting ClO⁻ was successfully developed. The response of the probe on test strip towards ClO⁻ was investigated, and the detection limit (0.1 μM) is low enough to meet the safety requirements in tap water. Furthermore, the quenching mechanism of Ex_g-C₃N₄ probe was also discussed.