Abstract: Time-averaged velocities of three-dimensional turbulent flow field in a cyclone separator were measured by Laser Doppler Velocimetry. The features in the flow field, especially in dust hopper,annular space and outlet tube, were discussed and analyzed. In the separation space, the outer vortex can be approximately regarded as quasi-free vortex and inner vortex as quasi-forced vortex. There were many secondary lognitudinal eddies in the entrance of the annular space, while a secondary lognitudinal flow existed near the roof of cyclone at a majority of azimuthes except the entrance. The distribution of time-averagel tangential and radial velocity was asymmetric in the annular space. The gas flux in the entrance was uneven along the axial direction. A secondary lognitudinal flow still existed near the roof of dust hopper. The distribution of time-averaged axial velocity in the outlet tube was very different from that in the separation space.