To realize the online analysis of heavy metal lead ion（Pb,2+,），a label-free aptamer electrochemical sensor was constructed with the hybridization indicator methylene blue（MB） as the electrochemical signal probe and the aptamer as the recognition element of Pb,2+,．On the surface of the gold electrode，the aptamer and its complementary sequence cDNA were successively modified to form a double strand，and then the MB adsorbed between the double strands produced a strong electrochemical signal．In the presence of Pb,2+,，the aptamer specifically captured Pb,2+, and released MB，resulting in the decrease of the electrical signal so as to realize the quantitative detection of Pb,2+,．Electrochemical impedance spectroscopy and cyclic voltammetry were performed to characterize the successful development of the constructed aptamer electrochemical sensor，while the feasibility of the sensor was proved by PAGE gel electrophoresis analysis．Differential pulse voltammetry was applied to record the electroactive signal of MB in Tris－HCl buffer solution．After optimization，the optimal experimental conditions of the aptamer electrochemical sensor were as follows：incubation time of cDNA：120 min；concentration of MB：1.6 mmol/L；incubation time of MB：120 min；incubation time of Pb,2+,：60 min．The linear response range for the sensor to Pb,2+, was 0.1－100 000 μg/L，and the limit of detection was as low as 33.4 ng/L．The results of the specificity test showed that the other interfered metal ions play a non-obvious effect on Pb,2+, detection．The proposed method was applied to the determination of Pb,2+, in milk samples and lake water samples．The spiked recoveries were in the ranges of 87.1%－115% and 106%－108%，with their relative standard deviations（RSDs） of 10%－13% and 5.0%－9.5%，respectively．The label-free aptamer electrochemical sensor constructed in this paper exhibits the advantages of low cost，sensitivity and rapidity，which is expected to be employed in the field analysis of Pb,2+, in the environment and food industry.