Conservation tillage is a primary strategy in dryland farming systems, that can significantly improve water productivity in dryland crops. However, the mechanism of conservation tillage drives optimization of root water uptake and evapotranspiration (ET) components to enhance efficient water utilization is unclear. Therefore, a two-year field experiment was carried out with spring wheat based on a long-term conservation tillage experiment in a semiarid region of Northwestern China to determine root water uptake, quantify evaporation and transpiration, and assess their relationship with yield and water use efficiency (WUE) under different tillage and straw management practices. The treatments were conventional tillage (CT), no tillage with no straw returning (NT), conventional tillage with straw returning (CTS), and no tillage with straw returning (NTS). Stable oxygen isotope (18O) analysis was used to determine root water uptake, evaporation (E) and transpiration (T). The results showed that wheat root absorbed water from greater depths as the growth stages advanced, and water was absorbed from deeper in the soil profile under NTS than other treatments. Transpiration changed greatly as the growth period advanced, with an initial increase, before then decreasing. The maximum transpiration occurred at jointing stage to flowering stage. Compared with CT, NTS, CTS, and NT decreased evaporation (E) and significantly increased transpiration (T) by 21.7%, 13.9%, and 7.2% in two growing seasons, respectively. Therefore, the transpiration to evapotranspiration ratio (T/ET) under NTS, CTS, and NT were 24.7%, 17.0%, and 11.1% higher than CT in two growing seasons, respectively. Our findings demonstrate that conservation tillage not only enhances root water uptake from deeper soil but also optimizes ET components by enhancing T while reducing E, thereby improving WUE in wheat. The integration of no-tillage with straw returning under NTS produced a synergistic effect that further optimized root water uptake and ET components, resulting in the greatest enhancements in both grain yield and WUE. Elucidation of this underlying physical mechanism advances the understanding of efficient water utilization in wheat under conservation tillage, thus providing insights for selecting appropriate conservation tillage in semiarid regions.