The invention relates to a supported metallocene catalyst and a preparation method thereof. The supported metallocene catalyst mainly comprises an inorganic silica silica-gel carrier, a polystyrene azo chlorine compound, a metallocene and a metallocene cocatalyst. The polystyrene azo chlorine compound serving as a sustained release agent is reacted with the metallocene to immobilize the metallocene in a skeleton network, and the metallocene is released in the supporting process and is supported by a carrier activated by alkyl aluminoxane, so that metallocene active centers are distributed uniformly, and a condition that effective active centers are reduced after the metallocene is combined with the cocatalyst because the local concentration of the metallocene active centers is too high is avoided. Therefore, by the method, the utilization efficiency of the metallocene in high price is improved, the production cost of the metallocene catalyst is reduced and the activity of the catalyst is kept.