Laser Doppler velocimetry (LDV) is one of the recent applied technologies in optical detection, and it has become an important research topic recently. In this research work, a previous developed Laser Doppler velocimetry system has been modified and applied to the tube flow rate measurement. We used optical fiber components as waveguides to make it easier to guide and focus the sampling light to tube flow. The scattered light was collected and coupled with the reference light to produce an interference beam. When the fluid flowed in the tube, the Doppler shift frequency according to the flow rate would exist in the interference beam. The Doppler shift frequency is calculated by using short-time Fourier transformation (STFT) algorithm to obtain the flows velocities. The tube flow contained the microparticles, therefore Mie scattering phenomena needed to be investigated. In the experiments, the 1 micron polystyrene suspension was used with a concentration of 1:50 and a peristaltic pump was used to pump the fluid flowing through the tube at the velocity of 5 mm/s, 10mm/s, 20mm/s, and 30 mm/s. The STFT algorithm programmed by matlab was used to acquire the spectrum and the variation of frequency. The measurement results confirmed that the particle flow rate has a linear relationship with the frequency of the STFT analysis. In this study, an LDV system has been established, which can measure the flow rate of tube particles by Doppler shift measurement and can be easily manipulated during the process.