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CO2 is the plant’s raw material for photosynthesis. In order to realize the precise regulation of CO2 gas fertilizer under different nitrogen levels，a fitting model for the photosynthetic rate of tomato was establish throughout its growth cycle. In this paper, the experiments were conducted with 3 CO2 concentrations and 3 nutrient liquid nitrogen treatments, and the environmental information and the net photosynthetic rate of the leaf were collected by the LI-6400 portable photosynthetic rate instrument. The prediction model of photosynthetic rate in the whole tomato growth stage was established by multiple linear regression. The correlation coefficient is 0.885 and the adjusted coefficient of determination was 0.782. The experimental results show that the model has high prediction accuracy, and can be used to guide the accurate regulation of CO2 gas fertilizer under different nitrogen content.