Dynamic laser speckle scattered by blood flowing in a vessel is studied in frequency domain. The power spectrum of the speckle field is derived based on single scattering approximation, and an algorithm is proposed to obtain the power spectrum from the speckle intensity. Results show that the spectrum is generally determined by three parameters: the ratio of static and dynamic speckle field, the characteristic frequency and the fixed doppler shift. Algorithm to get these parameters from a simulated power spectrum is presented, which shows a good accuracy. Procedures to measure the flowing velocity and the random movements based on the parameters are developed.