This study aimed to develop a nitrogen fertilization strategy for rainfed maize systems on the Loess Plateau, based on an optimal available nitrogen supply, to meet soil N availability with crop demand while minimizing groundwater quality damage from nitrate leaching. A three-year field experiment was conducted with varying planting density and N rate. The optimal relative yield was achieved at an integrated nitrogen nutrition index of 1.0 with an available N supply (ANS, mineral N fertilizer plus initial soil nitrate N) around 300 kg ha⁻¹, which represented the optimal level for minimizing soil nitrate residues without yield penalty. This ANS-based framework offers a practical strategy for sustainable N management in rainfed maize production under variable climatic conditions.