Abstract: Water content plays a crucial role in the soil K content and availability. In this paper, laboratory incubated experiments were carried out to study the effects of different water contents (0, 25% and 40%) and alternative drying-wetting condition on soil available potassium (K) change in red soil and yellow-cinnamon soil at a constant temperature (25±1)℃ for 120 days. Results show that compared with the control (dry condition), available K in red soil is increased significantly under two constant contents of 25% and 40%, and the increasing trend is increased with incubated time prolonged. While for yellow-cinnamon soil, available K for 25% and 40% water content treatments are lower than that of control. There are not any significant differences for the available K between the two constant contents of 25% and 40% for same soil, which indicate that the water film thickness of soil particle surface might not be the controlling factor to influence potassium availability in soils. Under the alternative drying-wetting condition, available K in red soil is not changed obviously, while available K is increased gradually in yellow-cinnamon soil, which indicates that the drying process promoting the release of inter-layer K of yellow-cinnamon soil. With the increase of times of the dry replace the humid, the K fixation capacity of yellow-cinnamon soil is decreased, while that of red soil is increased.