A novel assembly of CdSe/12ZnS core/shell quantum dots (CdSe/12ZnS QDs) and gold nanoparticles (Au NPs) was constructed for highly sensitive detection of nerve agent mimic diethyl cyanophosphonate (DCNP). Due to fluorescence resonance energy transfer (FRET) from CdSe/12ZnS QDs to Au NPs, the emission intensity of QDs was quenched. Thiocholine generated in situ from acetylcholinesterase (AChE) and acetylthiocholine (ATC) would destroy the assembly structure to recover the emission of QDs. However, diethyl cyanophosphonate (DCNP) could inhibit the activity of AChE, thus leading to the suppressed fluorescence recovery. Therefore, the detection of DCNP was achieved by evaluating fluorescence recovery efficiency of QDs. Optimally, a linear relationship between fluorescence recovery efficiency of QDs and logarithmic concentration of DCNP was observed in a range of 5.0×10 ^-9 mol/L to 5.0×10 ^-4 mol/L. As a result, the limit of detection (LOD) was deternimed to be as low as 5.0×10 ^-9 mol/L.