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 the lodging resistance, promoting the growth and development, and ultimately enhancing the yield and quality. The field experiments were conducted with two Qingke varieties —‘Kunlun 14’ and ‘Menyuanlianglan’—in Xining, Qinghai, 2021–2022. DAP was applied at five levels: 150, 225, 300, 375, and 450 kg·hm⁻² (designated as E1, E2, E3, E4, and E5, respectively). Agronomic traits, grain and straw yields, grain nutritional components, and straw forage characteristics were measured at maturity. The results showed that an appropriate amount of DAP increased the wall thickness, stem diameter, stem weight, and stem strength and promoted the lateral stem growth and development, thus improving the lodging resistance and straw yield of Qingke. At the same time, an appropriate amount of DAP promoted the development of roots and spikes, which was conducive to increases in grain number per spike and thousand-grain weight. Consequently, the grain yield was significantly increased, and grain and straw yields reached their highest levels under E3 treatment. Across two years, ‘Kunlun 14’ and ‘Menyuanlianglan’ showed the average grain yields of 4.71 and 4.11 t·hm⁻², and the average straw yields of 9.45 and 8.89 t·hm⁻², respectively. Moreover, the application of DAP facilitated starch accumulation in grains and reduced the content of hemicellulose, neutral detergent fiber, and acid detergent fiber in straw, while increasing the crude protein content and relative feeding value. Structural equation modeling indicated that DAP had a direct positive effect on straw yield, and grain number per spike, thousand-grain weight, and straw yield had significant positive effects on grain yield. The lodging rate during the grain-filling stage exerted direct negative effects on grain number per spike 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 and help improve the nutritional value of grains and the quality of straw as feed, being an effective cultivation measure to enhance the yield and quality of Qingke grains and straw.