Beauveria bassiana can colonize and promote the growth of many plants, especially improving the biotic/abiotic stress resistance of host plants. Elevated CO₂ is one of the key environmental drivers with far-reaching to plant growth, and it also has a regulatory effect on plant-microbial interaction. However, little is known about the effects and mechanisms of B. bassiana on plant growth and development under elevated CO₂ concentration. To clarify the effects and mechanisms of B. bassiana colonization on plant growth under increased CO₂ concentration, maize and B. bassiana were studied in this study as important species in agroecosystem, and a controlled experiment was conducted to study the effects of B. bassiana colonization on the growth and photosynthetic characteristics of maize plants under elevated CO₂ were studied in Open-top chambers (OTCs). The results showed that the root irrigation method facilitated the colonization of B. bassiana in maize, which promoted maize growth, and the maize treated with B. bassiana demonstrated increased plant height, leaf area, biomass, and weight per 100 seeds throughout the entire growth period under elevated CO₂ concentration. Furthermore, it was found that there were significantly differences in the number and morphological size of stomata in maize leaves among different groups under laser confocal microscopy, the number of stomata in the B. bassiana treatment was significantly lower than that in the control group, and the number of stomata was significantly increased under elevated CO₂ concentration, at all growth stages, at the same time, the length and width of stomata decreased at each growth stage when B. bassiana was inoculated with increased CO₂ concentration, and the length and width of stomata in B. bassiana treatment were the smallest; the increase of CO₂ concentration and colonization of B. bassiana had significant effects on the photosynthetic indicators of maize leaves, such as net photosynthetic rate and intercellular CO₂ concentration were significantly increased, and the transpiration rate and stomatal conductance were both decreased. This study indicated that the colonization of B. bassiana in maize plants with increased CO₂ concentration could enhance photosynthesis of the plants, and promote the growth of host plants under elevated CO₂ concentration. This study elucidated the effect and mechanism of B. bassiana in promoting maize growth, the findings provided a scientific basis for promoting the growth and improving the yield of maize. Moreover, this study provided technical support and theoretical guidance for the application of entomopathogenic fungi in the context of climate change.