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Since CO₂ is the fundamental substrate for photosynthesis, fluctuating concentrations have a direct effect on plant growth and metabolism. Accordingly, CO₂ enrichment within a certain range was found to improve photosynthesis, yields and the quality of plants. In order to further understand the underlying impact of CO₂ enrichment, this study employed an open-top chamber growth box model with the following two treatments: control treatment (CO₂ concentration: 380 ± 30 μL/L) and CO₂ enrichment (1200 ± 50 μL/L). The effects on leaf carbon assimilation, fruit yield and quality were subsequently determined. The net photosynthetic rate, intercellular CO₂ concentration, dry matter accumulation and soluble sugar content in the oriental melon leaves increased significantly on day 5 of CO₂ enrichment. Moreover, a significant increase in the activity of carbon assimilation-related enzymes Rubisco, RCA, FBPase and CA was also observed, with the upregulation of <i>CmRubisco, CmRCA, CmFBPase</i> and <i>CmCA</i> gene expression from day 15 of CO₂ enrichment. Thus, the yield per plant and content of soluble sugars and soluble solids in the fruit also increased significantly. These findings suggest that CO₂ enrichment has positive effects on oriental melon growth, increasing photosynthesis and the activity of photosynthetic carbon-assimilation-related enzymes and associated gene expression, thereby improving fruit yields and quality. These results provide a foundation for the CO₂ enrichment of oriental melon cultivated in solar greenhouses in autumn/winter and winter/spring.