Context: In arid regions, plastic mulch and drip irrigation are key drought mitigation strategies, yet their combined effects with deficit irrigation on spring wheat yield formation remain unclear. Objectives: The aim of this study was to investigate the combined effects of plastic mulching and deficit drip irrigation on topsoil hydrothermal conditions and spring wheat growth under arid conditions, evaluate the contribution of wheat growth to grain yield and water productivity, and identify strategy to simultaneously improve grain yield and water productivity among different treatments. Methods: A two-year field experiment was conducted using two planting patterns (non-mulching-M1, plastic mulching-M2), two irrigation regimes (border irrigation-I1, drip irrigation-I2), and four irrigation levels (W1.25, W1.00, W0.75, and W0.50, abbreviated as 125%, 100%, 75%, and 50% of full irrigation, respectively). Results: Plastic mulching increased topsoil temperature and moisture mainly before seedling emergence, thereby enhancing seedling population. Plastic mulching also reduced crop water stress index (CWSI), and increased leaf area duration (LAD) especially at jointing stage, thereby promoting pre-anthesis dry matter translocation (DMT). In addition, plastic mulch shortened the gradual grain-filling duration (T1) and increased grain-filling rate, and improve thousand kernel weight. Drip irrigation regime reduced water stress at booting but increased kernels per spike. Furthermore, drip irrigation also improved LAD especially at maturity, prolonged grain-filling duration and increased post-anthesis biomass. Compared with full irrigation, W0.75 increased CWSI and decreased LAD from grain filling to harvest, but enhanced grain-filling rate, thousand kernel weight and post-anthesis biomass, thereby significantly improving crop water productivity. Partial least squares path modeling (PLS-PM) indicated that LAD, DMT and yield components were strongly correlated with grain yield, while thousand kernel weight was closely linked to water productivity. Compared with control, plastic mulch combined with mild deficit drip irrigation improved grain yield, WPET and WPI by 18.15-45.25%, 30.91-63.96%, 48.79-126.68%, respectively. Conclusions: Plastic mulch enhanced pre-anthesis biomass particularly during seedling establishment, whereas drip irrigation regime increased kernels per spike and increased post-anthesis biomass. W0.75 increased grain filling rate and post-anthesis biomass. Among 16 treatments, I2M2W0.75 coordinated grain yield and water productivity within the experimental range.