An acetylcholinesterase biosensor for the rapid detection of paraoxon was developed based on composites of magnetic Fe3O4-Au nanoparticles(Fe3O4-AuNPs) prepared by chemical co-precipitation.This magnetic nanoparticle composites and carbon nanotubes(CNTs) were immobilized on a home-made magnet/glassy carbon electrode(MGCE) and served as matrix for immobilizing AChE.The chemical composition,magnetic properties and morphology of magnetic nanoparticles composite Fe3O4-AuNPs were characterized by X-ray diffraction,vibrating sample magnetometry and transmission electron microscopy,respectively.The electrochemical characteristics of home made MGCE and enzyme biosensor were characterized by electrochemical impedance spectroscopy(EIS),cyclic voltammetery,and differential pulse voltammetry(DPV).The determination of paraoxon using the biosensor were investigated by DPV.Under the optimal conditions，the inhibition efficiency of the immobilized AChE was proportional to the logarithm of paraoxon concentration over the range of 3.6×10-6-2.9×10-2 mol/L with a detection limit of 1.6×10-7 mol/L.The proposed method was successfully applied in the determination of paraoxon in real water sample with recoveries of 98.0%-107%.