Quaternary Ag-In-Ga-S (AIGS) quantum dot (QD) is considered a promising, spectral-tunable, and environmentally friendly luminescent display material. However, the more complex surface defect states of AIGS QDs resulting from the coexistence of multiple elements lead to a low (<60%) photoluminescence quantum yield (PLQY). Here, we develop a novel convenient method to introduce Z-type ligands ZnX₂ (X=Cl, Br, I) for passivating the surface defects of AIGS QDs to dramatically enhance the PLQY and stability without affecting the crystalline structure and morphology. Results show that the addition of ZnCl₂ during the purified process of AIGS QDs leads to a 3-fold increase of PLQY (from 28.5% to 87%). Impressively, the highest PLQY is up to a recorded value of 92%, which is comparable to typical heavy metal QDs. Exciton dynamics studies have shown that the rapid annihilation process of excitons in treated QDs is inhibited. We also confirm that the improvement in PLQY is a result of the effective passivation of the non-coordinating atom on the QD surface by building a new bonding between sulfur dangling and Zn²⁺. The realization of high PLQY will further promote the application of AIGS QDs in luminescent displays.