In arid and semi-arid regions, optimizing field mulching and nitrogen (N) management is crucial for winter oilseed rape production. This three-year study on the Chinese Loess Plateau investigated how mulching modes (film, straw, and no mulching) and N rates interact to influence yield and water productivity through coupled water-light-carbon processes. We developed a coupled "water-light" efficiency index to diagnose leaf physiological status and applied structural equation modeling to quantify driving mechanisms. Results showed that film mulching combined with moderate N (210 kg ha- 1) achieved a robust seed yield of 3762.7 kg ha-1, comparable to the highest N rate but with 25 % less fertilizer input, while attaining superior water productivity of 10.83 kg ha- 1 mm-1. Mechanistically, film mulching acted as a hydrological regulator by reducing evaporation and stabilizing root-zone soil water. This favorable environment allowed moderate N to function as a physiological activator, enhancing leaf chlorophyll content and photochemical efficiency without the excessive water depletion often caused by high N inputs. The proposed coupled index effectively captured this synergy, identifying physiological states with "low water cost and high photochemical return." Structural modeling confirmed that yield formation was driven by a cascade where stabilized soil water promoted leaf physiological function and canopy structure, ultimately maximizing carbon assimilation. Consequently, prioritizing film mulching with moderate N harmonizes water supply and physiological demand, offering a sustainable strategy for dryland agriculture.