Trichlorfon (TF) is one of the organophosphorus pesticides used widely in agriculture. The content of this paper includes the exploitation of dominant optical properties of the quantum dots consisting of a core and multilayer shell CdSe/ZnSe/ZnS (QD). A biosensor was fabricated on the basis of this QD for rapidly detecting the residues of trichlofon pesticide with concentrations of 0.01 ppm to 5 ppm. The measurements were carried out to examine the morphology of the QD structure and fluorescent properties such as transmission electron microscopy, x-ray diffraction, absorption spectroscopy and fluorescence spectroscopy. The linking mechanism among biological agents and the specificity of the acetylcholinesterase enzymes in hydrolysis reaction of acetylthiolcholine was applied to create the changes in surroundings, affecting the fluorescence of the QD. In particular, the mechanism of bioluminescence resonance energy transfer (BRET) is discussed to clearly explain the recombination of electrons and holes in the QD.