The method of fixed site field was adopted in the experiment for the study of potassium fertilizer application on the effect of dynamic change of potassium content in the soil of cultivated layer. The result showed that with potash application continuously for 7 years, it still showed a positive effect on corn yield. The result also showed that without potash application the available K content in the soil decreased significantly, the soil fertility was plundered by crop production like this. Without potash application the slow released K content in the soil decreased obviously, but with potash application it decreased slightly. With potash application at the rate of K 2O 112.5 kg/hm 2, the balanced coefficient of potassium content in the soil was 0.96.It means that the input and out put of potassium was nearly equal. With potash application at the rate of K 2O 225 kg/hm 2, the balanced coefficient of potassium content in the soil was 1.34. It means that the input of potash was more than that of out put, the potassium fertilizer has some surplus in the soil.