A 24-metal Cd(II)–Sm(III) nanocluster [Cd₁₈Sm₆L₉(OAc)₃₆] (CSN) was constructed from a new flexible Schiff base ligand. It can be used as a probe to detect 2,6-dichloro-4-nitroaniline (DCN, LOD: 0.67–0.75 μM), which is not influenced by the presence of other pesticides. The response behavior of CSN to DCN is expressed using a first-order equation 1/I_475nm + 1/I_645nm + 1/I_945nm = A[DCN] + B, and it is used to determine DCN concentrations in the extract of fruits. For example, for the concentration determination of DCN in carrot extract, the ranges of recovery and RSD are 96.11–103.50% and 0.95–4.12%, respectively.