With the aim to evaluate the appropriate upward flow velocity in UASB reactor, the separation efficiency of solid, liquid, gas in three-phase separator (TS) of USAB reactor was simulated via CFD technology based on Eulerian multi-phase model. Five upward flow velocities (UFV) as 0.50, 0.75, 1.00, 1.25, 1.50m/h were adopted in simulation for a typical UASB reactor. The result showed that the UFV had an obvious effect on the flow pattern inside the TS, the average velocity of liquid in the whole UASB reactor was around 3 times of the designed UFV, the average velocity of gas was 10% higher than that of the liquid, the average velocity of solid was lower and about half of the designed UFV, and the maximal solid UFV was in the same level with the UFV. The separation efficiency of solid and gas was higher in low UFV than that in the higher situation, while the difference was less than 2%. Site experiment showed a good agreement between the simulated data and measured one with a relative error within 8%.