Characteristics of the intensity fluctuation of the light back-scattered by flowing particles are studied. The influence of moving particles on back-scattered light is divided into phase modulation and amplitude modulation, and a new model is proposed that includes the total information of both axial and lateral components of velocity. Based on the new model, the two velocity components can be calculated according to the Doppler frequency shift and the transit time derived from the detected light intensity signals. The proposed model is experimentally verified using polystyrene particle suspension flow.