Near 100% photoluminescence quantum yield (PL QY) is the first target for designing luminescent quantum dots (QDs). Here, an alloyed QD with a core/shell structure of CdZnSeS/ZnS is synthesized and characterized by the feature of large core size (about 10 nm) and thin shell (around three ZnS monolayers). It exhibits an efficient green emission with the PL QY of 99.8% and ultra-fast radiative transition rate (k) of 4.16 × 10 s. It demonstrates that the excited state will be rapidly decayed to its ground state through a radiative channel and less vulnerable to non-radiative processes. Under a high pump intensity (254.65 μJ cm), the CdZnSeS/ZnS QD still owns a long biexciton decay lifetime (τ) of 1.05 ns and high biexciton QY of 25.1%, corresponding to a suppressed non-radiative Auger recombination. The CdZnSeS/ZnS QD based light-emitting diode exhibits a peak external quantum efficiency of 20.1% and brightness of over 10 cd m at 5.2 V. We believe that the CdZnSeS/ZnS QD not only provides a near-unity QY but also ensures its QD based light-emitting diode (QLED) with an excellent performance, which could be an excellent QD material for QLED applications.