Protective forests play an important role in maintaining the stability of arid oasis agroecosystems; however, water exchange and competition between shelterbelts and cropland under limited water availability remain poorly quantified. This study used stable hydrogen and oxygen isotopes to investigate irrigation-water redistribution and water-use interactions in a maize (<em>Zea mays</em> L.)–poplar (<em>Populus alba</em> L.) shelterbelt system in the Minqin Oasis, Hexi Corridor. The results showed that irrigation water was transferred from cropland to shelterbelts through both soil lateral movement and root-mediated uptake. Across four irrigation events, modeled transpiration-related water exchange between farmland and shelterbelts reached 84.49 mm, accounting for 19.56 % of applied irrigation water. Irrigation-induced lateral transfer into shelterbelt soil averaged 13.26 % of irrigation depth, with the largest contribution occurring in the 20–60 cm soil layer (5.49 %). Maize and poplar exhibited substantial overlap in water-source use, with a mean proportional similarity index of 73.35 %, indicating strong potential competition for soil water resources. These findings reveal that irrigation water redistribution links cropland and shelterbelts through coupled hydrological processes and highlight the need to jointly consider crop water demand and shelterbelt water consumption in arid oasis management.