Experiments were conducted to study the difference of potassium fixation capacity of soils under 13 year application of K fertilizer and wheat straw return in the northwestern region and northern region of China. The results showed that continuous application of K fertilizer and wheat straw return could reduce fixation rate of added K significantly compared with NP treatment. With the increase of K concentration in the surrounding solution, K fixation capacity of soils increased but the percentage of the total K in the solution fixed by soils reduced. K fixation rate in subsoil layer (20 ~ 40 cm) was greater than that in the top layer soil (0 ~ 20 cm), and it was more obvious in the northern region. K fixation rate by soils under double cropping region was higher than original soil but contrary under sole cropping region. K fixation capacity increased from the northwestern region to the northern region.