【Objective】Based on a long term fertilization experiment (1981 ~ 2012) in a double-cropping paddy field in Hunan Province, effects of long-term application of chemical potassium (K) fertilizer and incorporation of rice straw on yield and K uptake of rice and soil K supplying capacity in the double-cropping paddy field under intensive cultivation were explored. 【Method】 Rice yield, K uptake by shoot of rice, soil K pool including exchangeable K, non-exchangeable K and total K, soil K mineralogical characteristics and soil K supply capacity were determined. 【Result】 Results show that application of chemical potassium fertilizer and/or incorporation of rice straw increased the yields of rice grains and rice straw. Potassium uptake by the shoot of rice was significantly increased with rising K application rate. In terms of mean annual K uptake by the shoot of rice, the treatments displayed a decreasing order of NPK RS > NPK > NP RS > CK > NP. Long-term application of nil or insufficient K fertilizer, like in Treatments CK, NP and NP RS, caused depletion of exchangeable potassium, non-exchangeable potassium and total potassium in the topsoil. Long-term application of chemical K fertilizer and/or incorporation of rice straw not only increased the content of illite in soil clay, but also benefitted the transformation of poorly crystallized illite to well crystallized illite. The in-lab potassium saturation experiment on clay shows that K application increased the content of free illite and illite in clay of the soil, and expanded the diffraction peak area of total illite in K saturated soil, but reduced that of total V/CH. Moreover, in Treatments NPK, NP RS and NPK RS, compared with non-K saturation, K saturation reduced the diffraction peak area of total V/CH by an extent, lower than that in Treatments NP and CK, which indicates that long-term application of K fertilizer and incorporation of rice straw helps transform vermiculite into illite. The K quantity/ intensity (Q/I) curve parameters can be used to explain that the high K supplying capacity of the treatments of long-term application of chemical K fertilizer and/or incorporation of rice straw is attributed to higher labile K content (-△Kº), more specific K adsorption sites (Kx), higher available K intensity (AR_e^K), and higher tightly-held K content. However, these treatments are lower than those K absent treatments in potential buffering capacity (PBC^K), Gibbs free energy (-△G), and gapon cation exchange selectivity (K_G). 【Conclusion】 Long-term application of K fertilizers and/or incorporation of rice straw in addition to conventional fertilization in doubling rice cropping paddy field will not only significantly increase and stabilize yield of the rice, but also promote K uptake by shoot of the rice, improves soil K holding capacity, maintain soil potassium balance and build up soil K supplying capacity.