Water-soluble thioglycollic acid capped CdS quantum dots（QDs） were prepared using a simple method.CdS QDs were assembled on the surface of tin indium oxide（ITO） electrode through the electrostatic interaction between negatively charged thioglycollic acid on the surface of CdS and positively charged poly dimethyl diallyl ammonium（PDDA）.The CdS QDs modified ITO electrode showed a stable anodic photocurrent in the presence of triethanolamine（TEA） as an electron donor（or hole scavenger）.When the CdS QDs modified ITO electrode was immersed in Hg2+solution,an obviously enhanced photocurrent of CdS QDs was observed.The in-situ formation of HgS,a p-type semiconductor,on the surface of n-type CdS semiconductor led to the formation of a p-n junction,which facilitated the charge carrier transport and promoted electron-hole separation,triggered an obviously enhanced anodic photocurrent of CdS QDs.Effects of synthetic conditions such as molar ratio of Cd/S,pH value of reaction solution and reflux time on photoelectrochemical response of CdS QDs to Hg2+were investigated.Effects of pH value of the supporting electrolyte,applied potential and reaction time between CdS and Hg2+on photocurrent enhancement（I/I0） of CdS QDs by Hg2+were also investigated.Based on this,a sensitive Hg2+photoelectrochemical sensor was developed.The sensor showed a linear response to Hg2+in the concentration range of 4.0×10-8-2.0×10-5 mol/L,with a detection limit of 2.4×10-8 mol/L.The method was applied in the detection of Hg2+in lake water samples with recoveries of 98.3%–103.5%.