Near-IR spectroscopy holds great promise for non-invasive concentration measurements of blood on the basis of its potential for reagent-less, nondestructive, and noninvasive measurements. The main difficulty for determining absolute or even exact relative concentrations is the scattering behavior of the tissue. This leads to significant differences in the ideal Lambert Beer's law. In this paper, the approach of the Dynamic Spectrum in the frequency domain was proposed by Professor LI Gang etc. is shown, it is based on Photo-plethysmography (PPG) with fast Fourier transforms. The magnitude of fundamental wave of the pulse wave at each wavelength divided by the peak value of the pulse wave, get the natural logarithm of quotient at each wavelength and then the Dynamic Spectrum in the frequency domain is got. Evaluating only the pulsatile part of the entire optical signal, this approach is rather independent of individual or time changes in scattering or absorption characteristics of the tissue. Because of the noise and the resolution of the spectrometer, the Dynamic Spectrum is very difficult to get. In this paper, a series of measures is taken, and high-precision Dynamic Spectrum in the frequency domain is got with the experiment. The approach is verified. The advantage of getting Dynamic Spectrum in the frequency domain is analyzed, and compared with the Dynamic Spectrum in the time domain. The paper shows that the technique enables high precision measurement of changes in tissue absorbance caused by blood pulsation. It is very important in the non-invasive in vivo concentration measurement of blood.