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Rational plant density and fertilizer application are important crop management practices to improve maize grain yield. A two-year field experiment with maize varieties Suyu29 (SY29) and Suyu30 (SY30) was conducted with three plant densities (Low density, LD-6.0; Medium density, MD-7.5, and High density, HD-9.0 plants m−2) and three fertilization methods (no fertilizer, 0 F; one-off application of slow-released fertilizer, SF; and conventional fertilizer application, CF). Results indicated that the grain number and weight decreased with increasing plant density, and the grain yield initially increased and then decreased. The grain yields under SF with MD were the highest, being significantly higher than that under CF. The pre- and post-silking biomass accumulation and biomass remobilization from stems and leaves under SF were significantly higher than CF, thereby increasing its contribution to the grain and boosting grain yield. The grain-leaf ratios under SF and CF were similar across the three densities, and their source-sink ratios (SSRs) were similar at LD and HD. However, the SSR under SF increased by 8.4% for SY29 and 6.8% for SY30 relative to that under CF at MD. Our results showed that SF promoted biomass accumulation and remobilization to grain post-silking compared with CF at medium seeding density and delayed leaf senescence in the later stage, which increased the grain-leaf ratio, SSR, and harvest index and ultimately increased the grain yield. Consequently, selecting the appropriate density of 7.5 plants m−2 combined with one-off application of slow-released fertilizer is recommended to increase grain yield in the area under study. [ABSTRACT FROM AUTHOR]