Potassium metal batteries (PMBs) are a compelling technology for large-scale energy storage due to the abundance of potassium resources and high energy density. A vital obstacle to construct high-performance PMBs is developing superior cathode materials. Tannin, a plant polyphenol, holding many active sites for redox reactions, is an auspicious high-capacity cathode. However, the relatively poor conductivity and slight solubility in electrolyte deteriorate its capacity and cycle stability. Herein, the tannin is activated through a crosslinking reaction with polyaniline to obtain a composite cathode (i.e., ATN). It exhibits a high initial capacity of 172 mAh g−1 at 50 mA g−1, based on the electrochemical reaction mechanisms of embedding/releasing of K+ on C=O and PF6− on -NH-. Moreover, to improve the cycling performance, a graphene oxide modified separator is developed to effectively retard the ATN shuttle. As a result, the capacity retention is significantly enhanced, for example, 82% over 300 cycles at 50 mA g−1. From a practical aspect, a full PMB of K@KxPy||ATN is built up, displaying high energy/power density and superior cycling stability, which is a step forward in the development of PMBs.