From the view of natural resource utilization and protection, a study on the principle of potassium loss under different land use patterns in a red soil watershed was conducted in Zhejiang province of southern China. The results showed that K loss in a catchment with less vegetation cover and non soil conserving practices was highest, while lowest K loss occurred in catchment with vegetation conservation. K loss in an extensive farming catchment was higher than that of catchment in which comprehensive measurements for conservation were practiced. K loss was concentrated in the plum rainy season (May and June) and the typhoon rainy season (August), accounting for more than 90% of the total K loss. K was lost mainly in a particular form, accounting for more than 94% of the total K loss. The factors impacting on K loss were precipitation, rainfall erosion index, runoff and sediment.