Shelterbelt construction is considered an ecological solution to prevent sand erosion in the Xinjiang oasis. Shelterbelts, constructed from poplar (Populus), compete with crops for water, and thereby cause economic loss for farmers. We used direct inference of a stable oxygen isotope (δ¹⁸O) and the SIAR model to analyse the most common water uptake depth water uptake depth of cotton (Gossypium) at different growth stages under mulched drip irrigation, and poplar's water utilization strategy before and after irrigation. Results showed cotton primarily relies on soil water from a depth of 0-40 cm, taking up water from the 0-20 cm layer in bud stage, and from the 20-40 cm layer in boll stage. However, poplar mainly utilized soil water from below 40 cm, and the depth of soil water used by poplar gradually decreased after irrigation. We identified poplar roots in soil at 200-400 cm deep, extending 5-8 m into the cotton field. This reflected that poplar must compete for water and nutrition within this 5 m, especially the inner 3 m. This research increases our understanding of potential areas of water competition between poplar and crops and suggests reasonable management for shelter forest to maintain oasis ecological balance.