Determination of potassium can control the quality of compound fertilizer effectively.In this research,under suitable pretreatment，we established a quantitative method to determine potassium in the compound fertilizer by thermometric titration.The method to determine potassium was based on the precipitation reaction of sodium tetraphenylborate and potassium.Because of the exothermal reaction,the temperature changed obviously in the reaction chamber when equivalent point reached.The method which had the advantages of easy operation,fast and good sensitivity was feasible to supervise potassium in compound fertilizer.The conclusion showed that the result of thermometric titration was consistent with ion chromatography and potassium tetraphenylborate gravimetric method.The method had good accuracy and repeatability,and the recovery was above 97%.