Six field experiments were carried out to investigate the effects of potassium fertilizer management on watermelon yield and quality in the fields with very low, low, middle and high potassium concentration. The relationships between the leaf potassium concentration and yield and quality of watermelon were studied. The threshold potassium concentration in leaf was recommended to evaluate potassium nutrition in watermelon. Potassium concentration in watermelon young leaves was significantly positive correlated to watermelon yield (R²=0.600 8, P<0.01). Potassium content was 32.6 g/kg in watermelon young leaves when watermelon yield (91.39 t/hm²) was up to 90% of the highest yield. In soils with very low potassium levels (50 mg/kg and 70 mg/kg), K fertilization significantly enhanced watermelon yield. K content in young and old leaves at flowering stage, as well as increased K content in stem and root at flowering stage, fruit forming stage and fruit expanding stage. In soils with low potassium levels (84 mg/kg and 85 mg/kg), K fertilization significantly improved watermelon yields, concentrations of soluble sugar and soluble solid substance in fruits, while decreased titratable acid concentration in fruits. K concentration in young leaves, old leaves and leaves close to fruits was increased with the increase of K fertilizer quantity. Potassium concentration was increased in stems at fruit forming stage and fruit expanding stage. In soil with middle and high potassium levels (170 mg/kg and 260 mg/kg), the significant correlation was observed between the maximum watermelon yield and the highest K concentration in young leaves. K content of 32.61 g/kg was recommended as threshold in young leaves at flowering stage.