Abstract: Taking the coarse slime of Sihe Power Coal Preparation Plant as the sample, three spiral separators with the distance-diameter ratio of 0.4, 0.37 and 0.34 were designed on the basis of the traditional spiral separator. The influence of the distance-diameter ratio on the flow film thickness distribution of the spiral separator and the separation effect of the coarse slime was investigated. The results show that on the cross section of the spiral separator, the film thickness is thin at the inner edge and thin at the outer edge; the distance-diameter ratio has a great influence on the film thickness; the distance-diameter ratio of 0.34 is bigger than that of the inner edge, and the distance-diameter ratio of 0.4 is bigger than that of the outer edge; for coal samples with particle size ranging from 1.5 mm to 0 mm, the separation effect of the spiral separator with the distance-diameter ratio of 0.4 is better.