Understanding the environment-dependent stability and photoluminescence (PL) properties of advanced perovskite materials remains a challenge with conflicting views. Herein, we investigated the influence of the host matrix (poly(methyl methacrylate) (PMMA) and polystyrene (PS)) and atmospheric conditions (ambient and N2) on the PL properties of a CsPbBr3 perovskite quantum dot (PQD) using single-particle spectroscopy. Despite the same PL blinking mechanism, the PL properties of the PQD were considerably affected by the environmental conditions. The charge trapping and detrapping rates of the PQD were lower and higher, respectively, under ambient atmosphere than under N2 owing to surface defect passivation by oxygen. The frequency and rate of PQD decomposition were higher in the PMMA matrix than in the PS matrix under an ambient atmosphere. PS achieved superior PQD encapsulation owing to its higher affinity toward hydrophobic surface ligands because of its aromatic rings, thereby protecting the PQD surface from moisture and thus inhibiting decomposition.