Abstract: In arid areas, the rapid expansion of agricultural activities has caused a series of ecological and environmental issues such as vegetation degradation, declining groundwater levels, and land desertification, posing significant challenges to the stability and development of inland river basins. The development of agricultural water suitability is the key to ensure the water and land resources efficiency and ecological health in arid areas. The core is how to implement the scientific control of irrigation scale and agricultural irrigation water consumption. Taking the Shiyang River Basin as the study area, this paper analyzed the variations of high and low flow for surface runoff in the basin based on the anomaly percentage and the runoff modulus ratio coefficient. After estimating the non-agricultural irrigation water demand, the water balance model was employed to calculate the available surface water and groundwater resources specifically for agricultural irrigation, while also considering water transformation process. Based on this, a multi-objective planting structure optimization model was established to determine the water-saving and efficient crop planting structure. To determine the appropriate irrigation scale in arid areas, a comprehensive model was constructed by improving the water-heat balance model, which incorporates various factors such as the water transformation process, crop coefficient in different growth stages, agricultural water saving level, and crop planting structure. Lastly, the suitable irrigation scales of the Shiyang River Basin under different scenarios were explored by using this model. The results showed that the runoff in Shiyang River Basin presented obvious high and low flow variations during the studied period. Specifically, the basin’s total surface water resources amounted to 17.47×10⁸, 14.19 ×10⁸, and 12.25×10⁸ m³ during high, normal and low flow years, respectively. Regarding non-agricultural irrigation water demands, the critical and suitable ecological water demand scenarios were 3.43×10⁸ and 5.57×10⁸ m³, respectively. Following the fulfillment of water resources demand for non-irrigation sectors, and under the joint regulation of surface water and groundwater resources, the available water consumption for agricultural irrigation were 18.97×10⁸-21.75×10⁸, 14.75×10⁸-17.51 ×10⁸, and 12.31×10⁸-14.95×10⁸ m³ in high, normal and low flow years, respectively. With the improvement of water saving level, the decrease of groundwater recharge led to the decrease of irrigation water consumption in the basin. After optimizing the crop planting structure, 14.13% of irrigation water saving can be achieved by reducing the economic benefit of 2.94%. The suitable irrigation scale determined by the suitable irrigation scale calculation model in arid areas considering the water transformation process was consistent with the actual situation, which is more reasonable than the conventional method in the inland river basin. Based on this method, the suitable irrigation scales of Shiyang River Basin under the current condition were 27.73×10⁴-31.66×10⁴, 21.55×10⁴-25.76×10⁴, and 18.01×10⁴-22.03×10⁴ hm² in high, normal and low flow years, respectively. After improving the water-saving level and adjusting the planting structure, the suitable irrigation scale of the basin has increased. In 2020 (normal flow year), the actual irrigation area exceeded the critical suitable irrigation scale, necessitating reducing the irrigation area of 2.13×10⁴-6.34×10⁴ hm² to balance the water and soil resources in the basin. The research results provide a theoretical foundation for decision makers to formulate water-appropriate agricultural development plans at the macro level.