We report a facile and robust synthesis of ZnCdS core/shell quantum dots (QDs) with thick CdxZn1-xS (x = constant) uniform alloys as an intermediate shell which can provide effective confinement of excitons within the ZnCdS cores and ultrathin ZnS outermost shell to improve the stability by epitaxial growth at a relatively high temperature. The resulting nearly monodisperse ZnCdS/CdxZn1-xS/ZnS core/shell QDs have high photoluminescence quantum yield (near to 100%) and high color purity (full width at half maximum (FWHM) < 18 nm). More importantly, the ZnCdS/CdxZn1-xS/ZnS core/shell QDs have good chemical/photochemical stability and more efficient carrier transport performance compared with ZnCdS/ZnS core/shell QDs. Two types of QDs of ZnCdS/ZnS and ZnCdS/CdxZn1-xS/ZnS were incorporated into the solution-processed hybrid QD-based light-emitting device structure as the emissive layer. We find that the presence of the CdxZn1-xS shell makes a profound impact on device performances such as the external quantum efficiency and current efficiency. The corresponding light-emitting diodes exhibited a high EQE exceeding 18%, a peak current efficiency of 3.4 cd A-1 and low efficiency roll-off. Such excellent results of ZnCdS/CdxZn1-xS/ZnS-based QLEDs are likely attributable to the QD's high PL QY and very thin ZnS outermost shell which did not sacrifice the charge injection efficiency in QLEDs.