This study investigates the effect of CO2 fertilization before harvest using 800 mol·mol-1 from November 2021 to May 2022 and vase solution treatment using slightly acidic electrolyzed water (HOCl 5 µL·L-1, pH 5.0–6.5) after harvest of Hydrangea macrophylla ‘Moonstar’. The CO2 fertilization + HOCl treatment was the longest extension of the vase life and the largest total vase solution uptake. The vase solution treatment of HOCl influenced the vase life to a greater extent than by CO2 fertilization. In the CO2 fertilization treatment, the HOCl treatment reduced the number of bacteria on the end of stems by 54% compared to that achieved without HOCl treatment. It was higher on the relatively fresh weight and vase solution uptake in CO2 fertilization + HOCl treatment than that of control after 6 days of treatment. The CO2 fertilization + TW or HOCl treatments maintained the initial fresh weight for up to 6 days after treatment. There was no difference in the ΔE value, and chlorophyll contents of leaves and flower size did not differ in all treatments. Therefore, the initial sterilization on the end of stems after harvest was an essential factor for improving the quality of cut hydrangea flowers. The vase solution treatment of HOCl 5 µL·L-1 after harvest extended the vase life of cut hydrangea flowers.