Root system development fundamentally determines crop water and nutrient acquisition, but its functional coordination with aboveground parts remains poorly understood. To explore how root-shoot interactions regulate rainfed maize grain yield, quality, and resource utilization, a three-year field experiment was conducted in Northwest China, with three soil mulching patterns: non-mulching (NM), straw mulching (SM), and ridge–furrow film mulching (RM), and two nitrogen rates (0 and 180 kg N ha⁻¹). Beyond the traditional root-shoot ratio, eight novel indicators for describing growth-based, morphological, and material-based root-shoot synergy were proposed. Results showed that nitrogen application and soil mulching significantly enhanced aboveground and root growth, yield components, and optimized the dynamic balance of root-shoot ratios. Nitrogen application improved grain yield (GY) by 37.2 % and grain protein yield (GPY) by 45.2 % compared to N0. Relative to NM, SM and RM increased GY by 8.2 % and 26.7 %, and GPY by 8.3 % and 46.9 %, respectively. These practices also improved the capture and utilization efficiency of soil hydrothermal, nitrogen and radiation resources. The N180RM treatment obtained the highest GY (7464.7 kg ha⁻¹) and GPY (649.8 kg ha⁻¹), along with the highest resource use efficiency. Mantel tests demonstrated that root-shoot synergy was more closely associated with resource utilization efficiency than resource capture. The principal component analysis and structural equation modeling revealed that nitrogen fertilization had similar effects on aboveground and root systems, while soil mulching influenced root growth more significantly than aboveground growth. Among the three types of root-shoot synergy, growth-based synergy had the greatest impact on GY and GPY, which mainly depended on the ratio of root diameter to stem diameter. This study elucidated the internal mechanisms underlying high-yield and high-quality maize production through root-shoot synergy, and provided a scientific foundation for optimizing nitrogen fertilization and soil management in Northwest China.