<p>Clarifying the effect of postponing potassium fertilizer application on yield under nitrogen fertilizer reduction is of great importance for increasing yield and optimizing the nitrogen application system of Tartary buckwheat A two-year field experiment was carried out on high-yield Tartary buckwheat cultivar Daku 1. Four potassium fertilizer postponing treatments were set up (conventional nitrogen fertilizer + potassium as base fertilizer, control; reduce the application of nitrogen fertilizer 20%+ potassium as base fertilizer, T1; reduced nitrogen fertilizer 20%+70% potassium as base fertilizer + 30% potassium topdressing, T2; reduced nitrogen fertilizer 20%+50% potassium as base fertilizer + 50% potassium topdressing, T3). The 20% nitrogen reduction treatment significantly reduced average and maximum grain-filling rate, starch synthase activity, chlorophyll content, antioxidant enzyme activities, and root nitrogen metabolism related enzyme activity. Compared with control, T3 treatment significantly increased the above indexes and significantly reduced the malondialdehyde content. Compared with control, T3 treatment increased grain number per plant, grain weight per plant, 1000-grain weight, and yield by 35.45%, 26.05%, 8.73%, and 19.07%, respectively. In summary, the appropriate potassium fertilizer postponing (50%) can improve the Tartary buckwheat yield under nitrogen reduction condition. Therefore, this study can serve as a theoretical basis for improving yield and nitrogen saving cultivation of Tartary buckwheat. Notably, this experiment was carried out in Guizhou Province, China. Therefore, future studies should explore whether this technology is suitable for use in other producing areas of Tartary buckwheat.</p>