Abstract: As a core process of plant water transport, sap flow serves as a vital indicator of plant transpiration and water consumption, and its dynamics exert a significant impact on the stability of ecosystems. As one of the main afforestation species in the Mu Us Sandy Land, the water use status of Pinus sylvestris var. mongolica is directly related to the ecological stability of artificial forests. Despite its importance, the dynamics of Pinus sylvestris var. mongolica sap flow and its influencing factors remain unclear. To address this gap, we observed the dynamics of sap flow and meteorological factors during the period from February 2023 to January 2024 in the Mu Us Sandy Land. Based on structural equation modeling and boosted regression tree models, we identified the dominant influencing factors on sap flow and quantified their respective contributions. The results show that sap flow in Pinus sylvestris var. mongolica mainly occurred during the growing season from April to September. Hourly sap flow exhibited a bimodal pattern under different weather conditions, and the occurrence time and formation mechanism of the peaks vary accordingly. There was a 2-hour time lag effect between sap flow and the peaks of air temperature, wind speed, vapor pressure deficit, as well as the valley of relative humidity. In contrast, the response of sap flow to photosynthetically active radiation showed no significant time lag. The dominant meteorological factor influencing sap flow shifted from air temperature during the inter-growth period to photosynthetically active radiation during the active growth period, signifying a transition from multi-factor to single-factor control. Specifically, the relative contributions of air temperature to sap flow was highest, at 44.81%. These findings provide a crucial theoretical foundation for the strategic planting and management of Pinus sylvestris var. mongolica in semi-arid regions and offer valuable scientific insights into vegetation restoration in ecologically fragile areas.