As a kind of heavy-metal-free wide bandgap quantum dots (QDs), ZnSe has received extensive attention in the field of blue quantum dots and its light-emitting diodes in recent years. However, there is usually a large lattice mismatch at the core-shell interface of ZnSe based blue quantum dots, where defect states can be easily formed to capture charge carriers and thus deteriorate their optical properties. In this work, ZnSeTe / ZnSe / ZnSeS / ZnS quantum dots with Se / S graded-composition ZnSeS intermediate shells in radial direction were synthesized by replacing the homogeneous single-layer ZnSeS shell with the graded-composition ZnSeS shells. X-ray diffraction, steady-state photoluminescence spectroscopy, time-resolved luminescence spectroscopy, transmission electron microscopy and electroluminescence measurement techniques were used to study the effects of the different intermediate shells on the structure, morphology and optical properties of the synthesized quantum dots. The results show that, all of the synthesized quantum dots emit deep blue light (444. 5 nm) with narrow full widths at half maximum ( < 18 nm), and they also have uniform size and a zinc blende structure with high crystallinity. As the smoothness of the composition gradient in the ZnSeS intermediate shell along the radial direction is continuously improved, the optical properties such as photoluminescence quantum yield (PLQY) and color purity of the quantum dots are gradually enhanced. Among them, the quantum dots with the linearly graded ZnSeS shells formed by S atom diffusion have the narrowest full width at half maxium (15. 8 nm) and the highest PLQY (20. 7% ). The maximum external quantum efficiency and brightness of the light-emitting diode device prepared using the quantum dots with optimal fluorescence performance as emitting materials are 1. 8% and 750 cd / m2 , respectively. The quantum dot synthesis scheme and structural optimization strategy proposed in this study may contribute to the development of high-quality non-toxic blue ZnSe based core / shell quantum dots.