With artificial simulative washing test on different slope, the soil potassium loss by erosion in loess area was preliminarily studied. Results showed that the concentration of ionic K increases with the increase of slope. The relationships between slope and amount of K loss can be expressed by the exponential functions. The different forms of potassium were enriched in sediment. K mainly migrated with sediment in unsolvable forms, while slowly available K chiefly removed with soil.