Mulching and nitrogen (N) management are widely used to improve crop productivity in rainfed drylands, but their combined effects on stage-resolved water-N coordination remain insufficiently quantified. A three-season field experiment was conducted on rainfed winter oilseed rape (Brassica napus L.) on the Loess Plateau in China. Three mulching treatments-no mulching (NM), straw mulching (SM), and film mulching (FM)-were combined with five N rates: N0, 0 kg N ha(-1); N1, 70 kg N ha(-1); N2, 140 kg N ha(-1); N3, 210 kg N ha(-1); and N4, 280 kg N ha(-1). The growing season was divided into phenology-based windows to quantify stage evapotranspiration (Delta ET), aboveground plant N uptake increments (Delta N-uptake), and water-N coupling efficiency (WNCE). Seasonal dynamics were evaluated using Logistic cumulative curves, and the process-to-outcome pathway from water-nitrogen coupling to yield formation was examined using standardized path analysis. Compared with NM, FM increased seed yield and water productivity, while seasonal ET changed only modestly. The improvement was mainly associated with enhanced reproductive-stage WNCE and increased aboveground N accumulation during yield-sensitive windows. Model-derived rate curves indicated that FM reduced the temporal gap between ET and Delta N-uptake peaks around flowering. Standardized path analysis further suggested that yield improvement was mainly associated with the N uptake-mediated pathway rather than a direct ET pathway. Under the present experimental conditions, FM combined with 210 kg N ha(-1) (FMN3) was the best-performing treatment, achieving near-maximum seed yield with 25% less N input than N4. These findings indicate that coordinating mulching and N input can strengthen the reproductive-stage water-nitrogen coupling pathway in rainfed winter oilseed rape, although the broader sustainability of film mulching requires further evaluation.