Achieving synergy between water conservation and yield increase remains a central challenge in arid agriculture. While drip irrigation under plastic mulch (DM) is known to outperform traditional border irrigation under plastic mulch (BM) in maize production, the systemic mechanisms behind this advantage lack comprehensive explanation through long-term observational data. This study addressed this gap via an eight-year (2014-2021) fixed-site comparative field experiment in northwest China. Results demonstrate that DM improved the field micro-climate, increasing net radiation by 7.32% and soil water content by 21.60%. At the plant level, DM elevated the carbon-to-nitrogen ratio in roots, stems, and leaves, indicating enhanced indicating enhanced nitrogen use efficiency and stronger assimilate transport to grains. At the population level, DM boosted photosynthetic capacity, raising gross primary productivity and net ecosystem productivity by 13.05% and 15.58%, respectively. By regulating the "source-flow-sink" relationship, DM optimized the biomass partitioning pattern partitioning pattern, directing more photoassimilates towards grain filling. Structural equation modeling revealed that DM reconfigured the yield formation pathway and alleviated environmental stress limitations. Crucially, the leaf area index exerted the greatest total effect on fruit growth under DM-26.47% higher than under BM. Ultimately, the 12.85% yield increase under DM is attributed to the synergistic improvement of the field microenvironment, carbon-nitrogen metabolic efficiency, canopy photosynthesis, and biomass allocation, offering a scientific basis for reconciling water savings with yield gains in arid farmlands.