Abstract: In vegetable growing under protection, lack of carbon dioxide is one of the most important factors limiting photosynthesis. The application of 600～1200 μmol·mol^-1 CO₂ in the plastic house increased photosyntheitc rates and ratio of C/N, promoted both vegetative and reproductive growth of bottle gourd plants. Plant height, stem diameter, leaf thickness and leaf area increased by 3.90～19.48%, 11.58～27.37%, 38.46～69.23% and 26.09～49.38%, respectively. Average single fruit weight and yield were 4.05～19.62% and 8.65～19.47% higher than that in normal air. Primary female flower position was lowered by 2.6～4.0. Net photosyntheitc rate and the carboxylase activity of RuBP were greatest under CO₂ 1000μmol· mol^-1. The optimal concentration of CO₂ applied to bottle gourd in spring was about 1000 μmol·mol^-1.