To mitigate greenhouse effects, carbon dioxide reduction reaction (CO2RR) has been used as an efficient means of carbon reduction. In CO2 electrolyzer, CO2 deficiency can happen and degrade the reaction efficiency. Herein, an efficient and long-lived formic acid three-cell electrolyzer is used to study the effect of CO2 deficiency, by operating the electrolyzer from full CO2 supply to CO2 deficiency. In addition, the effects of various CO2 fluxes and concentrations on the electrolyzer current, acid concentration and lifetime are investigated. This study quantitatively reveals the impact of CO2 deficiency on current density, product selectivity, and electrolyzer lifetime. The findings indicate that current density decreases by 12.74 %, while CO2 conversion efficiency drops by 92.5 %, demonstrating a significant reduction in the reactivity of CO2 conversion to formate ions. Conversely, the hydrogen evolution reaction is enhanced. Prolonged CO2 deficiency (below 13 ml/min) can also lead to catalyst degradation, including separation and dissolution within the cathode catalyst layer, ultimately diminishing overall performance. Compared with the CO2 flux, the CO2 concentration exerts a more pronounced influence. To ensure the electrolysis efficiency, the carbon dioxide concentration should not be less than 80 %.