Abstract: To investigate the regulatory effects of 28-epihomobrassinolide combined with potassium dihydrogen phosphate on the agronomic traits, biological yield, and nutritional quality of silage corn, a field experiment was conducted using two cultivars, 'Dafeng 1407' and 'Beinong Silage 208'. Four fertilization treatments were established: no fertilization (CK), 28-epihomobrassinolide alone (FA), potassium dihydrogen phosphate alone (FB), and their combination (FC). The FA, FB, and FC groups each included five concentration gradients, resulting in 15 concentration treatments in addition to CK, with three replicates per treatment; their effects on the agronomic traits, biological yield, and nutritional quality of the two cultivars were systematically analyzed. The results showed that the combined application (FC) significantly increased plant height, stem diameter, and relative chlorophyll content (SPAD value) in both cultivars (p < 0.05). Specifically, under FC-III, the plant heights of 'Dafeng 1407' and 'Beinong Silage 208' reached 369.47 and 369.93 cm, respectively, which were highly significantly increased by 12.64% and 12.78% compared with CK (p < 0.01); stem diameters reached 37.70 and 37.96 mm, significantly increased by 15.18% and 15.97% compared with CK (p < 0.05); and relative chlorophyll contents (SPAD values) were 53.98 and 52.98, significantly increased by 18.89% and 16.69% compared with CK (p < 0.05), indicating that the combined treatment optimized plant structure and enhanced lodging resistance. In terms of biological yield, under FC-III, the fresh forage yields of the two cultivars reached 93 779 and 96 580 kg/hm², and the hay yields reached 42 200 and 43 461 kg/hm², respectively; both were significantly higher than those of CK (p < 0.05), demonstrating that the combined treatment effectively promoted dry matter accumulation and increased biological yield. In terms of nutritional quality, both cultivars had the highest relative feed value (RFV) under FC-III, reaching 181.64 and 198.55, respectively, which were significantly increased by 6.35% and 6.34% compared with CK (p < 0.05). Meanwhile, the contents of crude protein, starch, and dry matter increased, whereas those of neutral detergent fiber (NDF) and acid detergent fiber (ADF) decreased, indicating that the combined treatment optimized fiber composition and improved forage quality. Principal component analysis and membership function analysis revealed that the comprehensive scores of the FC group were higher than those of the other treatment groups, demonstrating the best overall regulatory effect. Furthermore, correlation analysis, principal component analysis, and regression and path analysis revealed that the combined application of 28-epihomobrassinolide and potassium dihydrogen phosphate synergistically drove increases in the yield and quality of silage corn by promoting stem thickening, maintaining high leaf photosynthetic efficiency, and optimizing photosynthetic product partitioning. These findings provide a theoretical basis for elucidating the physiological mechanisms underlying the combined application of plant growth regulators and foliar fertilizers, and offer technical support for improving the production efficiency of silage corn and optimizing the supply structure of the feed industry.