This paper introduces a method which can eliminate the interference of 232Th to 40K directly in the y energy spectra by using a standard mixed source of 232Th and 40K. The comparison between this method and the efficiency curve method indicates that the relative deviation is less than 3% for 232Th and less than 1% for 40K. The advantage of this method is that the detecting efficiency of 40K can be determined

and the interference of 232Th to 40K can be eliminated without knowing the efficiency calibration curve of the detecting system.