Abstract: The Huang-Huai-Hai Plain is one of the most important maize production regions in China. However, the excessive nitrogen application and suboptimal planting density have been the critical limiting factors on the grain filling and yield in maize. It is required to optimize the planting density with balanced nitrogen application for maize yield. Furthermore, the yield potential can depend mainly on the integrated performances of photosynthetic performance, grain-filling processes, and dry matter partitioning. This study aims to investigate the effect of the nitrogen application rate and planting density on the canopy light interception, grain filling, and yield of summer maize under the integrated drip irrigation and fertigation in the Huang-Huai-Hai region. Field experiments were conducted during 2023 to 2024. Two varieties, MY73 and JiYu 757 (JY757) were used as experimental materials. Two planting densities were set as 67 500 plants/hm² (D1) and 97 500 plants/hm² (D2). Three rates of nitrogen application were 0 (N0), 225 kg/hm² (N1), and 360 kg/hm² (N2). The results showed that the nitrogen application significantly increased the maize leaf area index (LAI) and SPAD values under the same planting density. While there was also increase in the canopy light interception, photosynthetic potential, and net photosynthetic rate, compared with the without N treatment. Light interception in the second canopy layer significantly increased by 14.49%. Nitrogen application was significantly improved the grain filling parameters. Specifically, the weight of the maximum grain filling rate (W_max) was enhanced by 21.46%. There was some increase in the active grain filling period (P), grain filling duration of middle stage and late stage (T2, and T3). Furthermore, the nitrogen application increased the grain yield by 32.99%. Meanwhile, the LAI, light interception and photosynthetic potential decreased significantly in the high plant density treatment (D2), compared with the normal plant density treatment (D1). The light interception in the second and third canopy layers increased by 14.58% and 52.89%, respectively, while there was the decrease in the SPAD values and net photosynthetic rate. The grain filling duration of middle and late stage was shortened, where the weight of the maximum grain filling rate decreased by 5.45%. Post-flowering dry matter accumulation and yield significantly increased by 15.86% and 24.50% in the D2 treatment, respectively. Moderate nitrogen application at 225 kg/hm² was significantly enhanced the weight of maximum grain filling rate under both planting densities, leading to the long active grain filling period, grain filling duration of middle and late stage. More efficient grain filling was ultimately promoted the kernel weight. The higher LAI was observed at silking stage and post-anthesis dry matter accumulation in the D2N1 treatment, compared with the D1N2 treatment (local conventional practice). Specifically, the light interception in the second and third canopy layers increased by 12.84% and 47.47%, respectively, with a 22.70% yield enhancement over the D1N2 treatment. Mantel test analysis showed that the summer maize yield was significantly positive correlated with these parameters, such as the photosynthetic potential, SPAD values, net photosynthetic rate, second canopy layer light interception, post-flowering dry matter accumulation, duration of active grain filling, and duration of rapid filling and slow filling. In conclusion, the nitrogen application and planting density shared the significant effects on the light interception in different maize canopy, grain filling parameters and yield. The increasing planting density (97 500 plants/hm²) and moderate nitrogen application (225 kg/hm²) were then beneficial to enhance the light interception, promoting grain filling and yield of summer maize.