Copper indium sulfide (CIS) quantum dots are emerging as promising materials for solar cells, deep-tissue bioimaging, and light-emitting devices, due to their inherently lower toxicity and excellent photoluminescence properties. However, they typically exhibit a broad emission linewidth (∼300 meV), making them lack of color and single-photon purity and thus less competitive with prototypical Cd-based nanocrystals. Herein, we report a strategy to narrow down the emission linewidth of CIS quantum dots from ∼ 300 meV to ∼ 120 meV via manipulation of Zn-alloying. The spectral narrowing was firstly observed in the epitaxial overgrowth of ZnS shell on the hexagonal wurtzite CIS quantum dots. Initially, the products exhibit two emission bands, a broad emission with linewidth of ∼ 300 meV and a narrow emission with linewidth of ∼ 120 meV. The broad emission is attributed to the recombination of a delocalized conduction band electron with a Cu-related localized hole, while the narrow emission originates from the band-edge exciton. Spectroscopic and structural analysis indicate that Cu+ for Zn2+ cation exchange prior to heteroepitaxial overgrowth of a ZnS shell is the key factor to promote the narrow emission, likely by suppression of hole localization on Cu+-sites. Precise manipulation of the Zn-alloying in CIS cores leads to a symmetric single band-edge emission with a photoluminescence quantum yield as high as ∼ 35% at 670 nm and linewidth as narrow as 120 meV. These nanocrystals are integrated into light-emitting devices, which exhibit a high turn-on voltage of 4.5 V and maximum radiances reaching over 1000 cd/m2.