Abstract : In classical laser Doppler (LD) perfusion measurements the zeroth and first order moments of the power spectral density of the LD signal lead to estimation of the concentration, speed and flow of red blood cells (RBCs). Non linearities of the moments in relation with RBC speed distributions, filters used in LD instruments and the signal to noise ratio have been analyzed. We developed a new method for fast simulation of the LD signal spectrum, based on analytical forms of Doppler shift probability distributions derived from assumed light scattering phase function. The method was validated by comparing the calculated spectra with results of Monte Carlo (MC) simulations. For the semi-infinite, homogeneous medium and the single Doppler scattering regime, the same accuracy is obtained by analytical calculations and MC simulations. The method allows for simulating the LD signal in time domain and furthermore analyzing the index of perfusion for the assumed light wavelength, optical properties of the tissue and concentration of RBCs. The method can also be used in the development of new algorithms for LD flowmetry and imaging.