Abstract: The effects and interactions of different straw return modes and planting densities on maize leaf–soil ecological stoichiometric characteristics during different fertility periods were studied to provide a theoretical basis for the application and promotion of straw return as a field technology for effective soil carbon, nitrogen, and phosphorus resource utilization as nutrients in the Ningxia Yellow River Irrigation Area. Using a two-factor spilt-plot experimental design, the primary plot comprised two straw return modes: H0 (no straw return) and H1 (complete straw crushing and deep plowing return). The subplot comprised three planting densities in the following order: D1 (67 500 plants·hm^₋₂), D2 (82 500 plants·hm⁻²), and D3 (97 500 plants·hm⁻²). The carbon, nitrogen, and phosphorus contents in the maize leaves and soil were measured under different treatments. The stoichiometric ratios and internal stability indices were calculated, and the influence of soil environmental factors on maize-leaf stoichiometry was analyzed. The results showed that compared with the H0 mode, the H1 mode significantly increased leaf organic carbon, leaf carbon∶phosphorus, and soil organic carbon. Compared with the D2 and D3 densities, soil organic carbon, total nitrogen, and leaf carbon significantly increased by 10.26%−20.37%, 9.14%−17.49% and 7.83%−18.25% at D1 density, respectively. Compared with H0D3, HID1 significantly increased leaf organic carbon, leaf carbon:phosphorus, and soil organic carbon by 21.27%, 14.06%, and 25.83%, respectively. The internal stability indices showed that the leaf carbon, nitrogen, and phosphorus internal stability was higher in the H1 (complete straw crushing and deep plowing return) mode than in the H0 (no straw return) mode, whereas the highest leaf carbon internal stability was observed in the low density (D1) treatment compared with those in the medium- and high-density treatments. The internal stability of leaf carbon, nitrogen, and phosphorus followed the order H_C>H_N>H_P, indicating that the plants had greater carbon regulation capability. Correlation analysis showed that leaf carbon was highly significantly and positively correlated with soil carbon, and leaf phosphorus was significantly positively correlated with soil phosphorus. Leaf carbon∶nitrogen was significantly positively correlated with soil carbon∶nitrogen. Redundancy analysis indicates that soil alkaline dissolved nitrogen was the primary factor influencing maize leaf stoichiometry. The composite scores of the affiliation functions indicated that the HID1 treatment had the best regulatory effect on the internal stability of maize leaves and soil ecological stoichiometry. Under the H1 (complete straw crushing and deep plowing return) mode in the Ningxia Yellow River Irrigation Area, a maize planting density of D1 (67 500 plants·hm⁻²) was favorable for increasing maize leaves and soil carbon, nitrogen, and phosphorus contents, and optimizing ecological stoichiometric ratios, which led to the effective use of soil nutrient resources. This study systematically revealed, for the first time, the synergistic effects of straw return and planting density in the Ningxia Yellow River Irrigation Area.