Abstract: Taking the two-year old potted Prunus sibirica seedlings as test materials, and using CIRAS-2 photosynthetic system, this paper studied the CO₂ response process of P. sibirica photosynthesis in semi-arid loess hilly region under eight soil moisture conditions. The CO₂ response data of P. sibirica were fitted and analyzed by rectangular hyperbola model, exponential equation, and modified rectangular hyperbola model. Meanwhile, the quantitative relationships between the photosynthesis and the soil moisture were discussed. The results showed that the CO₂ response process of P. sibirica photosynthesis had obvious response characteristics to the soil moisture threshold. The relative soil water content (RWC) required to maintain the higher photosynthetic rate (P_n) and carboxylation efficiency (CE) of P. sibirica was in the range of 46.3%-81.9%. In this RWC range, the photosynthesis did not appear obvious CO₂ saturated inhibition phenomenon. When the RWC exceeded this range, the photosynthetic capacity (P_{n max}), CE, and CO₂ saturation point (CSP) decreased evidently. Under different soil moisture conditions, there existed obvious differences among the three models in simulating the CO₂ response data of P. sibirica. When the RWC was in the range of 46.3%-81.9%, the CO₂ response process and the characteristic parameters such as CE, CO₂ compensation point (Γ), and photorespiration rate (R_p) could be well fitted by the three models, and the accuracy was in the order of modified rectangular hyperbola model > exponential equation > rectangular hyperbola model. When the RWC was too high or too low, namely, the RWC was ＞81.9% or ＜46.3%, only the modified rectangular hyperbola model could well fit the CO₂ response process and the characteristic parameters. It was suggested that when the RWC was from 46.3% to 81.9%, the photosynthetic efficiency of P. sibirica was higher, and, as compared with rectangular hyperbola model and exponential equation, modified rectangular hyperbola model  had more applicability to fit the CO₂ response data of P. sibirica photosynthesis under different soil moisture conditions.