Background: The dry bulb of Fritillaria thunbergii Miq. is a traditional Chinese medicine, and is the leading product of the geo-authentic crude drugs ‘Zhebawei’ in Zhejiang Province. Fritillaria thunbergii requires more potassium (K) than nitrogen and phosphorus, while the response of F. thunbergii to K has rarely been studied. Objective and Methods: To evaluate the K nutritional status of F. thunbergii, a 2-year field experiment was conducted under six K application rates (0, 40, 80, 120, 160, and 200 kg K2O ha?1) with two F. thunbergii cultivars (ZB1, narrow-leaf; ZB2, broad-leaf). Results: The bulbus Fritillariae Thunbergii (BFT) yield increased to a plateau with more than ~120 kg K2O ha–1. The BFT quality met the standard of Pharmacopoeia of P. R. China with more than 40.0 kg K2O ha–1. Leaf K concentration was significantly correlated with BFT yield, and was more sensitive to K application rate as compared to those of stems and flowers. Potassium application extended the duration of rapid leaf-biomass accumulation and increased the accumulation rate. Two cultivars had similar response patterns to K application, but ZB2 had higher K partial productivity, leaf-biomass, and leaf K concentration than ZB1. Conclusion: 108.4–128.0 kg K2O ha–1 was the optimal K application range for BFT yield. The potassium nutrition index (KNI) model established based on leaf K concentration has biological significance and can be used for dynamic diagnosis of K nutritional status of F. thunbergii with both accuracy and simplicity.