Abstract: Abstract: In this paper we synthesized CdSe quantum dots (QDs) double capped by mercaptopropionic acid (MPA) and mercapto salicylic acid (MSA) in aqueous phase, optimized the synthesis condition of the QDs and characterised it, discussed the toxicity of the QDs and applied it to label the salicylic acid receptor of mung bean seedlings. The results showed that when the pH was 11.30, the mole ratio of Cd:Se:MPA:MSA was 1:0.5:2.5:1, the reaction temperature was 90℃ and refluxed for 45 min, the fluorescence intensity of the QDs was best; the double capped QDs was well-distributed and the particle size of which was about 3-4nm. Compared to the MPA single capped QDs, the UV-VIS absorption spectrum of the double capped QDs showed a red shift, the fluorescence strengthened obviously and the excitation and emission wavelength ex/em of which was 365/540nm. The toxicity of the double capped QDs was lower than the single capped QDs; the label experiment demonstrated that the salicylic acid receptor of mung bean seedlings exists in the root mainly, precious few in the stalk, and some in the cotyledon