Abstract: The Black soil region of Northeast China is the main soybean producer,which is very important for guaranteeing the supply of vegetable oils to China.The effect of potassium (K) fertilization and rainfall patterns on soybean grain yield was investigated based on the crop rotation experiments in the National Observation Station of Hailun Agroecology System in the Chinese Academy of Sciences.Soybean grain yield was impacted by crop rotation patterns,rainfall patterns and rate of K fertilizer application in the study area.Soybean grain yield under continuous soybean was significantly decreased by 5.5% and 26.6% compared with soybean-maize rotation and soybean-maize-wheat rotation.The application of K fertilizer could regulate significantly the soybean grain yield in continuous soybean,and an increased trend of soybean grain yield with the increased rate of K fertilizer application was observed.However,soybean grain yield under soybean-maize-wheat rotation did not increase with increased rate of K fertilizer application when K fertilizer application rate was over 149 kg·hm^-2,indicating that the lower rate of K fertilizer application could be applied in soybean-maize-wheat rotation.K fertilizer could ameliorate the negative effect of wet growing season on soybean grain yield.Therefore,the application of K fertilizer could increase significantly the soybean grain yield under continuous soybean.