Lead ion (Pb²⁺) detection is critically important in environmental protection and health management. In this work, we developed a simple signal-enhanced colorimetric sensor for the detection of Pb²⁺ based on the peroxidase-mimetic property of gold nanoparticles (AuNPs). When a certain concentration of Pb²⁺ was added to a solution of DNAzyme-modified AuNPs, aggregation was triggered, and the result was an enhancement of the peroxidase-mimetic activity of AuNPs. Then, the chromogenic reaction of 3,3′,5,5′-tetramethylbenzidine (TMB) by the catalyst of AuNPs was used for the sensitive UV-Vis and colorimetric detection of Pb²⁺. When a higher concentration of Pb²⁺ was added, the greater amount of aggregation of AuNPs resulted in the enhancement of the UV-Vis adsorption of the solution at 652 nm, with a deepening of the blue color of the solution. After optimization of the experimental conditions, a linear relationship between the absorbance of oxidized TMB at 652 nm and the logarithm of Pb²⁺ concentration was obtained, which had been divided into two parts (25 pM to 2.5 μM, and 2.5 μM to 250 μM). The detection limit was as low as 10 pM. The satisfactory specificity and rapid response of the sensor showed that it has promising application for the detection of Pb²⁺ in real samples.