Abstract: Influence of length-diameter ratio and half cone angle of drum of horizontal screw decanter on separation performance was researched by use of numerical simulation method. 3D model building and meshing of horizontal screw decanter were introduced as well as numerical simulation process. The research results show that separation effect and separation efficiency of horizontal screw decanter is the best when the length-diameter ratio of drum is 4.3; coal slurry is squeezed stronger and it is good for dehydration of coal mud with the best seperation efficiency of the horicontal screw decanter when the half cone angle of drum is 9°.