Using graphene sheet(GS) as a matrix connecting bovine serum albumin to label microcystins(MCLR)(BSA-MCLR),an electrochemical immunosensor was developed for the determination of MCLR with AuNPs as a signal probe. Scanning electron microscopy(SEM), transmission electron microscopy(TEM) and UV-Vis absorption spectroscopy were used to characterize the synthetic nanomaterials. Cyclic voltammetry was used to investigate the electrochemical properties of the modified electrodes. Analyte MCLR was competitively combined with immobilized BSA-MCLR to form anti-MCLR. After the immune reaction, AuNPs were electrochemically oxidized into AuCl4- at a constant potential. Then the AuCl4- ions were reduced into Au with the peak current as the detecting signal via cathodic potential scan of differential pulse voltammetry(DPV). Under the optimal conditions, the linear range was from 0.1μg/L to 50μg/L with a detection limit of 0.05μg/L. The reproducibility, stability and selectivity of the immunosensor were investigated．Compared with the enzyme-labeled probes, the AuNPs labeled probe could have the determined process finished with low cost, high stability and excellent detection result.