Abstract: Topic detection and tracking (TDT) aims to develop a series of technologies for event based information organization, and hierarchical topic detection (HTD) is a new task of it. Through a series of large-scale evaluations, TDT has become a hot problem for worldwide research in the fields of natural language processing, especially in information retrieval. In this paper, an effective method of topic detection focusing on the features of events is proposed, and an arithmetic named MLCS is also offered to organize topics into hierarchical structures. The methods proposed are very effective, and score second in the HTD evaluation of TDT2004.