A series of well-defined azide polymers, poly(styrene-co-4-vinylbenzyl azide) with different content of azide groups, were synthesized by RAFT polymerization. The single-chain polymeric nanoparticles were then facilely prepared via single polymer chain collapse and intramolecular crosslinking reaction of the azide polymer in a dilute solution at room temperature under UV irradiation within several minutes. The polymeric nanoparticles were characterized with 1H NMR, GPC, DSC, TEM and DLS measurements, and the mean diameter of the nanoparticles was evaluated to be 5.5 ± 0.8 nm in the dry state. Furthermore, the polymeric nanoparticles containing azide groups were successfully used to prepare the single-chain polymeric fluorescent nanoparticles via click reaction with N-propargyl carbazole.