Abstract: Effects of potassium fertilization on dry matter yield and root system of Avena sativa cv. Qingyin No.1 were studied to optimize the potassium application for oats production in a potassium deficient area. The results show that the optimum fodder and protein yields of the cultivar are 29575.0 kg/ha and 2099.8 kg/ha respectively at the blossom stage under the N 54.75 kg/ha, P2O5 51.75 kg/ha and K2O 40 kg/ha fertilization. Relation between dry matter, protein yield and K application can be described as Y=a+bK+cK2. Under the K2O 40 kg/ha treatment, the plant height, tiller, root length and root number of the cultivar are the highest, are about 184.5 cm, 3.22 till/plant, 15.90 cm and 26.17 roots/plant, respectively. Under the 20 kg/ha K2O fertilization, diameter of stem and root weight are the highest, 0.585 cm and 0.540 g/plant, respectively. There are significant correlations between above-ground, under-ground yields and fodder, protein yields.