Dynamics of potassium in root-zone and non-root-zone red paddy soils under rapeseed-rice rotation was studied through a rhizobox equipment. Results showed that soil water soluble K and exchangeable K in root-zone soils were reduced first at the early stage of rapeseed growth. Along with plant growing and K uptake, soil water soluble K in the inner (0~20 mm), middle (20~40 mm) and outer (40~60 mm) parts of the non-root-zone were moving towards to the root zone. Soil exchangeable K were transformed into water soluble K, thus decreasing gradually. In the early stage of rice growth, water-logging promoted diffusion of soil water soluble K from non-root-zone to root zone and transformation of exchangeable K into water soluble K. Along with growth of rice plant and K uptake, soil exchangeable K in each part decreased significantly. In the late stage of rice growth, plant K uptake reduced in intensity. Soil water soluble K in the root zone dropped to a certain extent and then stopped declining, while soil water soluble K in the non-root-zone kept on diffusing towards the root-zone. During the whole rotation cycle, the treatment (with cropping) and CK (without cropping) were not much different in nonexchangeable K in the root-zone and non-root-zone. It revealed that the plant first absorbed K in the root zone, of which the K reserve was replenished with K gradually diffused from the non-root-zone. The closer to the root zone, the greater the contribution to K uptake by plant. Within one rotation cycle, soil exchangeable K and water soluble K are the main forms of potassium available to the plant, and little nonexchangeable K could be absorbed.