The present invention relates to a process for preparing an activating support for metallocene complexes in the polymerization of olefins comprising the steps of: I) providing a support consisting in particles formed from at least one porous mineral oxide; II) optionally fixing the rate of silanols on the surface of the support; III) functionalizing the support with a solution containing a metallic salt; IV) heating the functionalized support of step c) under an inert gas or hydrogen; V) oxidizing the support of step IV by treatment under N2O and then under oxygen; VI) retrieving an active support having a controlled number of OH groups. That activating support is used to activate a metallocene catalyst component for the polymerization of olefins.