The palaeo-atmospheric CO2 concentration (pCO2) variation in the Yumen, Gansu Province during the middle Cretaceous has been reconstructed using the newly established plant photosynthetic gas exchange mechanistic model, and the results show that the pCO2 values are in the range of about 550–808 ppmv. The present pCO2 values are higher than the pCO2 results (531–641 ppmv) of the previous study according to the Recent standardization of the stomatal ratio method, and much lower than the pCO2 results (882–1060 ppmv) according to the Carboniferous standardization of the stomatal ratio method. The present pCO2 variation is not only within the error range of GEOCARBⅡ and GEOCARB Ⅲ but also is similar to the reconstructed results based on the biochemistry and carbon isotope models. Besides, the present Brachyphyllum specimens were collected from four consecutive horizons of the upper Zhonggou Formation of the Hanxia Section, and the reconstructed pCO2 exhibits the reconstructed pCO2 exhibits a decline trend during the late Aptian to early Albian. This decline variation is probably associated with the Oceanic Anoxic Events (OAE1b) and the Cold snap event. With the combination of pCO2 during the Albian to Cenomanian recovered by the plant photosynthetic gas exchange mechanistic model, the pCO2 showed a prominent increase during the late Aptian to early Cenominian, which indicates a response to the greenhouse warming during the middle Cretaceous. Therefore, the mechanical model of the plant photosynthetic gas exchange shows a relatively strong accuracy in the reconstruction of the pCO2 and can reflect a strong relation between the atmospheric CO2 concentrations and climatic events.