The sources of noise in dynamic spectrum (DS) method and their corresponding frequency characteristic wereanalyzed in order to improve the signal to noise ratio (SNR) of DS method. The processes of DS data in frequency domain were reviewed by means of energy, then the harmonic waves of DS data were taken into account in the DS signal and some experiments were done to test whether the modified method works well. In addition, corresponding experiments were carried out to seek the relationship between the SNR and the number of harmonic waves, and to determine how many harmonic waves should be involved in order to get the best SNR in DS method. Results showed when harmonic waves were used in the method properly, the modified method can distinguish DS more precisely. And it was also showed that as the number of harmonic waves increased, the correlation coefficient of DS data from different volunteers became smaller at first and then bigger later, while the correlation coefficient of DS data from different sampling site of the same volunteer kept increasing all the time. When the number of harmonic waves was set to 5, the correlation coefficient of DS data from different volunteers goes from 0. 737 52 to 0. 736 76, while the one from the same volunteer goes from 0. 994 16 to 0. 995 33, which means the modified method can reflect the information about blood component more precisely than the old ones, and thus the SNR of DS reaches the highest.