A double cavity type steel slag treatment equipment by barrel method employing mandrel support comprises left and right barrel bodies, flow branching pan, mandrel, feed funnel. Between the left and right barrel bodies is disposed a flow branching pan, over which is mounted a feed funnel and below which is mounted a retaining ring. The mandrel is fixedly connected to the left and right barrel bodies and flow branching pan, supporting bearings are mounted at two ends of the mandrel, left and right barrel bodies have inner and outer portions, inner barrel body is made of grid section, one end of grid section is fixed on the end cover of outer barrel body through an insertion opening made of supporting rings, the other end of grid section is joined with the supporting ring fixedly connected on outer barrel body.