Water is a vital macronutrient and an important environmental factor, and is required for plants to conduct photosynthesis, which insufficient or excessive soil moisture will negatively affect. The soil water deficit is a key factor affecting plant physiological activity, restricting plant productivity and vegetative restoration. Rosa xanthina L. is one of the most widely distributed types of secondary vegetation in mountainous and hilly region in North China. Because of its high level of ecological adaptability, resistance to stress, and ecological functions, Rosa xanthina shows great potential value for development and application to the reconstruction of vegetation in the ecologically fragile Loess Plateau. Thus far, previous studies of Rosa xanthina have focused on such features as its root structure and soil reinforcement capabilities, the structure and ecological adaptations of its associated community, the morphology and anatomical structure of its leaves, and its photosynthetic characteristics. However, few studies have investigated the response relationship between photosynthesis and soil moisture, or the effective use of soil moisture by Rosa xanthina in arid habitats. The objectives of this study are to explore both the photosynthetic response to soil moisture and light and their quantitative relationship, and to attempt to identify the threshold effect of soil moisture on photosynthesis in Rosa xanthina L. In addition, this study aims to determine the species' photosynthetic productivity classification using physiological photosynthesis data. This new information will provide references that will help further our understanding of the water use and photophysiological characteristics of Rosa xanthina under drought stress. Three-year-old Rosa xanthina seedlings which were collected from the nursery and then potted in buckets were taken as materials in the semiarid loess hilly and gully regions. Sufficient water was provided to each seedling to saturate the soil in the bucket two days before the determination of photosynthetic parameters, and the soil water content was then monitored with no further watering. We measured soil water content and the photosynthetic parameters every two days until the seedlings withered. Using a CIRAS-2 portable photosynthesis system, the light reaction processes of Rosa xanthina plants were measured under eight soil moisture conditions found in the semiarid loess hilly region. The result showed that the net photosynthetic rate (P_n), quantum yield (Φ), and water use efficiency (WUE) exhibit clear threshold response characteristics. Higher photosynthetic rates (P_n) and WUE were observed in Rosa xanthina leaves when the relative soil water content (RWC) increased from 36.2% to 81.2%, and there was no significant photoinhibition of photosynthesis. When RWC was approximately 66.5%, Φ and P_n peaked, exhibiting strong potential energy utilization. The transpiration rate (T_r) and WUE peaked when RWC respectively were 81.2% and 44.5%. Based on the quantitative relationship between RWC and the plants' physiological photosynthetic characteristics, the RWC values measured when P_n, Φ, T_r, and WUE reached their respective maxima, and when RWC corresponded to the inflectional values of intercellular CO₂ concentration (C_i) were designated as points of differentiation. Thus, we propose a metric for the grading of soil moisture availability according to its effect on Rosa xanthina photosynthesis. Soils with an RWC lower than 36.2% are considered "low productivity and low efficiency" grade, those with an RWC of 36.2%-44.5% are considered "low productivity and high efficiency" grade, those with an RWC of 44.5%-66.5% are considered "high productivity and high efficiency" grade, those with an RWC of 66.5%-81.2% are considered "high productivity and medium efficiency" grade, and those with an RWC greater than 81.2% are considered "low productivity and low efficiency" grade.