The invention relates to a volute of a centrifugal fan. The volute comprises a front cover plate, a rear cover plate and an annular middle ring wall. The front cover plate and the rear cover plate are connected together through a continuous smooth transition warp surface of the middle ring wall. The front cover plate and the rear cover plate are provided with front air inlet holes and rear air inlet holes respectively. The projection of the circle centers of the front air inlet holes in the plane of the rear cover plate coincides with the circle centers of the rear air inlet holes. The volute is characterized in that the edge outline of the front side of the middle ring wall is formed by connecting a straight line AB, a straight line BC, a curve CD, a curve DE, a curve EF, a curve FG, a curve GH, a curve HI, a curve IJ, a curve JK, a curve KL, a curve LM, a straight line MN, a straight line NO and a straight line OP which are sequentially in smooth transition, and the edge outline of the rear side of the middle ring wall is the same as the edge outline of the front side. Compared with the prior art, by means of the special design on the middle ring wall of the volute, the structure is more reasonable, the internal smoothness of the volute is improved, and the static pressure is increased; and meanwhile, the noise lowering effect is achieved, and the volute is suitable for the small size.