In a catalytic FCC cracking unit comprising a vertically disposed riser housing a reaction zone, the improvement comprising at least one device for redistribution of catalyst inside said reaction zone, the catalytic cracking unit being equipped at a plane with feedstock injectors, and an upper plane, with mixed temperature control (MTC) injectors, wherein said device has the general shape of a continuous ring adjacent the riser wall and along the inside periphery of said wall, said device creating a restriction in the cross section of the riser ranging between 30% and 45%, having a profile section at least partially rectilinear, and being located a) either in a zone encompassed between the plane of the feedstock injectors and the plane of the MTC injectors, b) or at a height of between 0.1 and 5 meters above the plane of the MTC injectors.