In order to investigate the change of soil available potassium in farmlands in Jiangsu Province and its influential factors, the data of the long-term monitoring points of farmlands in 2008 and 2015 were used to analyse the temporal and spatial variation of soil available potassium by classical statistical and geostatistical methods, and multifactor analysis was used to study the effects of parent material, climate, cropping system, fertilizer and their interaction on the change of soil available potassium. The results showed that the average content of soil available potassium was 107.9 mg/kg in 2015, insignificantly different with 2008. The contents of soil available potassium in coastal areas decreased obviously in 2015 compared with 2008, and the largest decrease appeared in Yancheng city, decreased by 10.0-30.5 mg/kg; While the largest increase appeared in Xuhuai region, increased by 10.1-20.5 mg/kg. Soil parent material, climate and potash fertilization had significant effects on the content of soil available potassium from 2008 to 2015. Soil parent material and climate dominated the change of soil available potassium under the extreme-low input level of potassium fertilizer, but potash fertilization dominated that under the low and higher input levels of potassium fertilizer, indicating potash fertilization is the main measure to improve soil available potassium content.