A simple colorimetric biosensor based on the different adsorption ability of gold nanoparticles（AuNPs） on single-strand DNA（ssDNA） and G-quartet was presented to determine Pb2+ concentration in water.The guanine-rich ssDNA can be easily adsorbed onto the surface of AuNPs preventing them from aggregation in high ionic strength solution.In the presence of Pb2+,the conformation of ssDNA was changed from random coil to rigid G-quartet structure which could not bind to AuNPs,leading to the aggregation of AuNPs,accompanied by color changing from red to blue and red shift of the maximum absorption wavelength.Under the optimal conditions,the absorption ratio of A630/A520 was proportional to Pb2+ concentration in the range of 0.1-10 μmol/L with a detection limit（3σ） of 50 nmol/L.There were almost no interference from other metal ions.With the advantages of high sensitivity and selectivity,simplicity of preparation and manipulation,lowcost,fast response（within 1 min）,the presented method is very suitable for real-time and on-site application in environmental protection.