The residues of the high toxic organophosphorous can cause chronic poisoning.At present,the detection of organophosphorous residue needs complicated operation,time consuming and high cost.Here,we harvested total RNA of mosquito by Trizol methods,constructed cDNA library,and got the exon of acetylcholinesterase.The acetylcholinesterase from culex was expressed by Bac-to-Bac system,then the enzyme was purified using Ni-sepharose.The crude and purified enzyme was tested and verified by SDS-PAGE.The enzyme activity was determined by Ellman methods.It will be of great significance to biosensors in organophosphorous pesticides detection.