A high activity magnesium-based supported catalyst component useful in a catalyst system for the compolymerization of ethylene and alpha-olefin and a process for preparing the catalyst component is described. In the process, alkoxysilane ester is contacted with a halogen-substituted silane to form an organic silicon complex. Optionally, the organic silicon complex is contacted with an aminosilane compound to form an organic silicon complex containing nitrogen. The organic silicon complex containing nitrogen or the organic silicon complex is contacted with a transition metal compound to form an organic silicon complex containing transition metal. The organic silicon complex containing transition metal is then contacted with a substituted aromatic ring nitrogen compound to form a fourth reaction complex, which is then contacted with a magnesium-based composite support that has been prepared in situ by reacting metallic magnesium with an alkyl or aromatic halide to form the catalyst component.