The present invention relates to a transition metal half metallocene catalyst with a noble structure for preparing syndiotatic styrene polymer having high activity, superior stereoregularity, high melting point and broad molecular weight distribution and a process for preparing styrene polymer using the same. The present invention provides a half metallocene catalyst having a single nucleus structure, in which a transition metal in Groups 3 to 10 on the periodic table is connected to a cycloalkanedienyl group or its derivative forming 5-coordinate bond on a side thereof and to any one of triethanolamine, N-alkyldiethanolamine and N-dialkylethanolamine group, all of which have a plurality of binding sites and high steric hinderance, on the other side thereof. The noble metallocene catalyst according to the present invention is useful for preparing highly syndiotatic vinyl aromatic polymer with broad molecular weight distribution and high activity.