Abstract: This study aimed to explore the effects of potassium application rates on the yield, dynamics of nutrient accumulation, and photosynthetic and thermal physiological characteristics of summer maize, identify the optimal potassium rates, and provide a reference for the scientific application of potassium in summer maize. Field experiments were conducted during two consecutive growing seasons (2021 and 2022) in Wenxian County, Henan Province, China. The maize variety ‘Longyu 369’ was used as the test material. The detailed potassium fertilization treatments (K₂O) in the two growing seasons were 0 (K0), 40 (K40), 80 (K80), 120 (K120), and 160 (K160) kg·hm⁻². Plant height, leaf area, leaf SPAD value, photosynthetically active radiation, canopy temperature, antioxidant enzyme activity, and potassium utilization efficiency of summer maize were determined and analyzed. The results showed that the application of potassium fertilizer significantly increased the summer maize yield. Compared with the K0, potassium application increased the maize yield by 4.9%−6.6% and 7.4%−9.6% in 2021 and 2022, respectively, and the optimal economic potassium rates were 80 and 87 kg·hm⁻², respectively. Moreover, potassium application significantly increased plant height, leaf area, and leaf SPAD values in summer maize. Logistic equation fitting showed that the optimal treatments for total potassium absorption and maximum accumulation rate were K80 and K120, respectively. Compared with K0, potassium absorption and accumulation increased by 22.5% and 28.3% on average for K80 and 29.2% and 37.5% on average for K120, respectively. Additionally, to maximum potassium accumulation the time of maximum rate of occurrence could be shortened by 3–4 d. This research also showed that the suitable potassium application rates of 80–120 kg·hm⁻² could effectively enhance the photosynthetically active radiation fraction, reduce canopy temperature, and increase the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT). Therefore, reasonable potassium application could significantly increase the yield, promote growth and potassium absorption utilization, and enhance the photosynthetic and thermal physiological and antioxidant enzyme activity characteristics of summer maize. The recommended potassium rates based on the experiments are between 80 and 120 kg·hm⁻².