A novel electrochemical immunosensor for microcystin-（leucine-arginine）（MCLR） was developed based on gold electrode modified with Au nanoparticles（AuNPs）.The AuNPs were prepared by the sodium citrate reduction method,and characterized by transmission electron microscopy（TEM） and UV-vis absorption spectroscopy.1，6-hexanedithiol（HDT） molecules were modified onto the surface of a gold electrode to form self-assembly monolayer, and then the AuNPs were assembled on the self-assembly monolayer of HDT. The immunosensor was constructed by immobilizing the antibody of microcystin-（leucine-arginine）（anti-MCLR） on the modified electrode. The surface morphology of the AuNPs modified electrode and the electrochemical behaviour of the immunosensor were characterized by scanning probe microscopy，cyclic voltammetry and electrochemical impedance spectroscopy（EIS），respectively.MCLR and horseradish peroxidase conjugated MCLR（MCLR-HRP） competitively combined with immobilizing anti-MCLR. The determination of MCLR using the immunosensor was investigated by differential pulse voltammetry（DPV）.Under the optimized conditions，a linear range from 0.01μg/L to 25μg/L and a detection limit of 0.005μg/L were obtained by DPV method.The reproducibility，stability and selectivity of the immunosensor for MCLR were investigated. The proposed method was applied in the determination of MCLR in real water samples with recoveries of 100%-102%,and the determination results of MCLR were consistent with those obtained by HPLC method.