This invention relates to a vanadium catalyst system for preparing rubber elastomers. This invention introduces an activation promoter into the traditional vanadium catalyst system to form a new active catalyst system, which is composed of vanadium compound, cocatalyst and activation promoter. The vanadium catalyst system can be used in ternary polymerization of ethylene, propylene and diene, and has such advantages as high catalytic activity, low consumption and easy removal of vanadium during post treatment.