The dsDNA-selective fluorescent-dye-based DNA damage assay was developed for DNA-encoded library (DEL) synthesis. For the various DEL synthesis conditions, the assay was validated through cross-checking with high-performance liquid chromatography (HPLC) analysis, and the fact was confirmed that the usage of a specific ratio of organic solvent can critically induce DNA damage. Also, the applicability of the assay was confirmed through the screening of the DNA-damaging condition of the on-DNA amide coupling reaction and Pd-catalyzed on-DNA N-arylation reaction.