Abstract: Carbon capture and storage (CCS) technology plays an important role in reducing global carbon emission. However, CCS project is facing many obstacles, particularly the leakage from carbon dioxide (CO2) storage sites severely threatens ground ecological systems. In this study, a manual control device was designed and constructed, on an agricultural ecological system threatened by leakage from CO2 storage sites, to simulate the scenarios of geologically stored CO2 leaking from underground to ground. The changes of several important parameters of an agricultural ecological system (AES) were observed and recorded so as to evaluate impacts of underground stored CO2 leaking on an AES, and then to establish the tolerable threshold value. The results showed that maize growth was negatively affected under different CO2 leakage amounts. In general, maize grew worse when CO2 leaking flux increased. For example, seedling of maize was severely hampered, both plant height and leaf amount decreased with increasing of CO2 leaking flux. Biomass for both underground and above ground with CO2 leaking was significantly less than that of control, photosynthesis of maize was affected, and soil pH value droped. As a result, the tolerable threshold value of CO2 leakage from geologically stored sites is 500-2 000 g/(m2·d).