This study sought to clarify the effects of potassium (K) fertilizer application on the growth and K requirement of Festuca arundinacea and determine the appropriate K fertilizer application rate for its normal growth. A pot experiment was conducted to test the plant growth and K uptake of Festuca arundinacea response to different K fertilizer rates (0, 100, 200, 300, and 400 mg K2O kg−1) in two K levels (medium K and low K) of soils. Potassium fertilizer application and soil significantly affected plant growth, K uptake and K concentration in the shoot, and K uptake in the root. Low-K soil and without K fertilizer application increased the root-shoot ratio. Increasing K fertilizer application enhanced K distribution in the shoot. The recommended optimum K fertilizer rate to obtain 80% of the maximum shoot biomass in medium-K and low-K soils was 88.9 mg K2O kg−1 and 71.1 mg K2O kg−1, and the corresponding critical K concentration of shoot was 17.9 g kg−1 and 14.4 g kg−1. Appropriate K fertilizer application could promote plant growth and K uptake. An optimized K fertilizer management strategy and K diagnostic concentration should be carried out according to soil K levels during the establishment phase of Festuca arundinacea.