The purpose of this study is to explore the response of mini-cucumber seedlings to double CO₂ concentration. Two kinds of mini-cucumbers were used as materials to study the effects of CO₂ enrichment on the morphological indexes, seedling indexes and photosynthetic parameters of greenhouse mini-cucumber seedlings. CO₂ enrichment increased the net photosynthetic rate, intercellular CO₂ concentration and light saturation point of cucumber,and decreased stomatal conductance, transpiration rate, light compensation point and CO₂ compensation point. CO₂ enrichment significantly increased the stem diameter and plant height of mini-cucumber seedlings, increased leaf number and leaf area, significantly increased dry matter accumulation, increased net assimilation rate and seedling index, and increased root-shoot ratio. The enrichment of CO₂ increased the photosynthetic performance and seedling quality of mini-cucumber. Genqing 27 is more sensitive to CO₂ enrichment than Duofu.