High planting density in maize affects yield through photosynthetic capacity and assimilate transport in the lower canopy. A 2-year field experiment with three high-yield maize varieties (LY66, MC670, and JK968) at four planting densities (75,000, 105,000, 120,000, and 135,000 plants/ha) showed that increased density initially increased yield but then decreased it. MC670 had the best adaptability to high-density conditions. High density reduced single-leaf area, chlorophyll content, and photosynthetic rate, leading to lower biomass accumulation in the lower canopy. However, it improved presilking biomass transfer, especially in the lower canopy. Lower canopy photosynthesis and biomass were strongly linked to grain yield under high-density conditions. High density also delayed ear development due to reduced glucose and fructose in ears, leading to lower kernel numbers and yield loss. The study highlights the importance of lower canopy photosynthesis for yield under high-density planting. Strategies to optimize lower canopy structure and stimulate ear development are needed to improve maize yield under high-density conditions. The findings provide insights for future maize breeding to enhance yield and food security.High planting density in maize affects yield through photosynthetic capacity and assimilate transport in the lower canopy. A 2-year field experiment with three high-yield maize varieties (LY66, MC670, and JK968) at four planting densities (75,000, 105,000, 120,000, and 135,000 plants/ha) showed that increased density initially increased yield but then decreased it. MC670 had the best adaptability to high-density conditions. High density reduced single-leaf area, chlorophyll content, and photosynthetic rate, leading to lower biomass accumulation in the lower canopy. However, it improved presilking biomass transfer, especially in the lower canopy. Lower canopy photosynthesis and biomass were strongly linked to grain yield under high-density conditions. High density also delayed ear development due to reduced glucose and fructose in ears, leading to lower kernel numbers and yield loss. The study highlights the importance of lower canopy photosynthesis for yield under high-density planting. Strategies to optimize lower canopy structure and stimulate ear development are needed to improve maize yield under high-density conditions. The findings provide insights for future maize breeding to enhance yield and food security.