The crack width of sludge return crack for the three-phase separator (TS) plays a key role in the separation process. To evaluate the effects of crack width on the separation efficiency of solid, liquid, gas in TS of USAB reactor, a 3-D CFD simulation was adopted based on Eulerian multi phase model. Five typical crack width settings as 80mm, 95mm, 110mm, 125mm, 140mm were investigated for a typical UASB reactor. The result shows that the average flow speed of three phases in the TS region was much higher than the UASB’s designed up-flow speed, and the flow speed of solid phase exceeded that of gas and liquid. Circulation pattern turbulence dominated the liquid flow in TS region. The crack width had more obvious effect on solid phase than on gas and liquid. The separation efficiency was much effective for solid phase in five cases through one platewhile only 60% for gas. The separation efficiency of gas and solid phases became lower with a wider crack gap. With the supplement of precipitation layer in the top of TS, the solid phase could separate completely, while the highest separation efficiency for gas was obtained with gap width of 95mm.