Large-area hypoxia of urban landscape water often causes the emissions of harmful gases in summer, which not only reduces its sensory effects, but also brings a potential threat to aquatic ecosystem and human health. This study explored the dose effects of calcium peroxide (CaO) on inhibiting harmful gases emissions and restoring the scenic effect (including visual sense and olfactory sense) of anoxic/anaerobic landscape water system. The results indicated that the emissions of HS, CO and CH from the anoxic/anaerobic water system were obviously inhibited in the reactors with CaO additions and the effect was positively correlated with the CaO dose. Meanwhile, the concentrations of total chemical oxygen demand (TCOD) and soluble sulfide (S), and turbidity in the overlying water (the water-layer above the sediment-water interface) were also decreased in the reactors dosed with CaO. The reason was ascribed to the improvement of the anoxic/anaerobic condition in the water system and the increase of the species richness, bacteria count and aerobic microorganism abundance in sediment. Furthermore, 0.12 kg-CaO m-sediment was selected as the optimal dose, which was based on considering the inhibiting effect of the harmful gases emissions, comprehensive influence and costs. Compared with control check (CK, the reactor without adding CaO), the optimal dose of CaO could reduce 75.10% CH, 81.02% CO and 100% HS in gases, and decrease 81.52% S, 42.85% TCOD and 84.01% turbidity in the overlying water. In conclusion, all the dosages of CaO could improve the anoxic condition of water system and 0.12 kg-CaO m-sediment was the optimal dose in inhibiting harmful gases emissions, which could keep an excellent water quality in this simulation experiment. Therefore, this study may provide a feasible method and the optimal dose for inhibiting the emissions of harmful gases and restoring the scenic effect in the similar anoxic/anaerobic landscape water.