【Objective】This study aimed to evaluate the effects of long-term straw and straw ash application on rice yield and soil potassium (K) supply capacity.【Method】Based on a long-term field experiment initiated in 2010 in Jinxian, Jiangxi Province, four treatments were established: no straw return and no fertilizer (CK), chemical fertilizer alone (NPK), full straw return combined with chemical fertilizer (NPK+RS), and full straw ash return combined with chemical fertilizer (NPK+RA). Soil samples (0-20 cm) were collected after the late rice harvest in 2023 to determine total K and different K fractions, K content at different adsorption sites of clay minerals, K release kinetics, and quantity/intensity (Q/I) relationship, and rice yield was analyzed accordingly.【Result】Fertilization significantly increased rice yield and soil K content. Under equal NPK nutrient inputs, NPK+RS and NPK+RA increased average annual yields of early rice by 5.22% and 3.53%, and late rice by 3.68% and 2.41%, respectively, compared with NPK. Water-soluble K showed little variation among fertilization treatments. Compared with NPK, non-exchangeable K in NPK+RS and NPK+RA increased by 13.16% and 17.12%, respectively. NPK+RA significantly enhanced exchangeable K, effective K, and total K by 5.41%, 2.87%, and 3.09%, respectively, and its exchangeable and effective K contents were 7.88% and 4.99% higher than those under NPK+RS. Fertilization significantly increased K contents at clay mineral adsorption sites. Compared with NPK, K contents at the mineral surface (p sites), edge (e sites), and interlayer (i sites) increased by 45.07%, 10.09%, and 6.27%, respectively, under NPK+RS; the corresponding increases under NPK+RA were 49.46%, 16.97%, and 11.91%. During K release, no significant differences were observed among fertilization treatments in the rapid-release stage. In the slow-release stage, compared with NPK, cumulative K release increased by 19.69% and 9.07%, and the release rate increased by 19.66% and 9.09% under NPK+RS and NPK+RA, respectively, with NPK+RS showing significantly higher values than NPK+RA. In addition, Q/I analysis indicated that both NPK+RS and NPK+RA optimized soil K supply capacity and intensity more effectively than NPK.【Conclusion】Under equal NPK nutrient inputs, straw and straw ash return combined with chemical fertilizer were superior to chemical fertilizer alone in improving rice yield and enhancing soil K supply capacity. Considering the adverse environmental impacts of straw burning, direct straw return combined with chemical fertilizer is recommended for preferential promotion.