Water pollution accidents, such as the Flint water crisis in the United States, caused by lead contamination have raised concern on the safety of drinking water distribution systems. Thus, the routine monitoring of lead in water is highly required and demands efficient, sensitive, cost-effective, and reliable lead detection methods. This study reports a label-free colorimetric nanosensor that uses unmodified gold nanoparticles (AuNPs) as indicators to enable rapid and ultra-sensitive detection of lead in environmental water. The 8–17 DNAzyme was truncated in this study to facilitate the detachment of single-stranded DNA fragments after substrate cleavage in the presence of Pb2+. The detached fragments were adsorbed over AuNPs and protected against salt concentration-induced aggregation. Accordingly, high Pb2+ would result in rapid color change from blue to pink. The established sensing principle achieved a sensitive limit of detection of 0.2×10–9 mol/L Pb2+, with a linear working range of two orders of magnitude from 0.5×10–9 mol/L to 5×10–9 mol/L. The selectivity of the nanosensor was demonstrated by evaluating the interfering metal ions. The developed nanosensor can serve as a substitute for the rapid analysis and monitoring of trace lead levels under the drinking water distribution system and even other environmental water samples.