We investigated the exciton decay dynamics of CsPbBr3 perovskite quantumdots (PQDs) through an X-type ligand passivation process. 1-Dodecanethiol (DDT), as an X-type ligand, covers Br vacancies of PQDs and then the photoluminescence quantum yield (PLQY) sharply improved from 76.1% to 99.8%. Impressively, after passivation, the photoluminescence (PL) lifetime of PQDs decreased from 3.16 ns to 2.42 ns, contrary to the PLQY increase. To clarify this phenomenon, we observed exciton decay dynamics by varying the temperature. Thereby, we found that shallow traps from Br vacancies not only reduce the PLQY but also induce a longer lifetime related to the nonradiative exciton decay leading to an increase in the lifetime. Our results are novel and important in a way that we provide a systematic understanding of the exciton decay dynamics by combining two key concepts together: (1) near unity PLQY via ligand engineering and (2) temperature-dependent photogenerated excitons.