A new method for the determination of potassium in compound fertilizer by ultrasonic extraction using inductively coupled plasma atomic emission spectrometry was established. Both the new method and the traditional method in the national standard Determination of potassium content for compound fertilizers(GB/T 8574—2010)were used to determine the potassium reference material and 10 samples of compound fertilizers with different potassium contents and source fertilizers. The t-test result showed that there was no significant difference between the results of ultrasonic extraction and traditional boiling extraction，and there was no significant difference between the results of ICP method and traditional gravimetric method. The recovery of the new method was 99.33%，and the relative standard deviation of 6 parallel determinations was 0.89%. The new method has the advantages of convenience，high accuracy，high precision and wide applicability.