Abstract: The irrational application of diammonium phosphate (DAP) not only leads to resource waste but also affects the normal growth and development, yield, and quality of Qingke in alpine regions. Therefore, optimizing the DAP application rate is crucial for improving lodging resistance, promoting growth and development, and ultimately enhancing yield and quality. The field experiments were conducted in Xining, Qinghai, in 2021–2022, using two Qingke varieties: Kunlun 14 and Menyuanlianglan. Five levels of DAPwere applied: 150, 225, 300, 375, and 450 kg·hm⁻² (designated as E1, E2, E3, E4, and E5, respectively). Agronomic traits, grain and straw yield, grain nutritional components, and straw forage characteristics were measured at maturity. The results showed that an appropriate amount of DAPincreased wall thickness, stem diameter, stem weight and stem strength; promoted lateral stem growth and development; improved lodging resistance; and increased straw yield. At the same time, an appropriate amount of DAP promoted the development of roots and ears, which was conducive to increases in grain number per panicle and thousand-grain weight. Consequently, grain yield was significantly increased, and grain and straw yields reached their highest levels under E3 treatment. Across two years, the average grain yields of Kunlun 14 and Menyuanlianglan were 4.71 t·hm⁻² and 4.11 t·hm⁻², respectively, while their average straw yields were 9.45 t·hm⁻² and 8.89 t·hm⁻², respectively. Moreover, it facilitated starch accumulation in grains and reduced the contents of hemicellulose, neutral detergent fiber (NDF), and acid detergent fiber (ADF) in straw, while increasing crude protein content and relative feeding value (RFV). Structural equation modeling indicated that DAP had a direct positive effect on straw yield, whereas grain number per panicle, thousand-grain weight, and straw yield all had significant positive effects on grain yield. In contrast, the lodging rate during the grain-filling stage exerted a direct negative effect on grain number per panicle and thousand-grain weight. In conclusion, under the conditions of this study, the recommended application rate of DAP for Qingke is 300−375 kg·hm⁻². A moderate reduction in fertilizer input (conventional reduction of 16.67%−33.33%) can promote the growth and development of Qingke plants, help improve the nutritional value of grains and the quality of straw as feed, and bean effective cultivation measure to enhancethe yield and quality of Qingke grains and straws.