Abstract: Objective To construct the theory of the fluid state of the blood flow of symmetrical narrow parts of artery and the distribution of the red blood cell mass. This theory is the basis of revealing the formation mechanism and the abundant connotation of the spectrum of doppler ultrasound signals from blood flow. Methods The methods used in the research include the observation of the fluid state of similar ditches, and scientific verification with mathematical theory. Results The consequence of the research includes the establishment of the theoretical model which explains the fluid state of the blood flow of the symmetrical narrow parts of artery, and the summary of the distribution rules of the numbers of the red blood cell in the red blood cell mass according to its flow velocity. Conclusion The research results indicate that in this essay, not only the theoretical basis for the explanation of the formation mechanism and the connotation of the spectrum of doppler ultrasound signals from blood flow, but also the references for the explanation of the fluid state of other parts of the artery are provided.

Key words: spectrum of doppler ultrasound signals from blood flow, fluid state of the blood flow, distribution rules of the red blood cell mass