Internet has become a major medium for information transmission, how to detect hot topic on web, track the event development and forecast emergency is important to many fields, particularly to some government departments. On the basis of the researches in the field of topic detection and tracking, we propose a model for hot topic discovery that will pick out hot topics by automatically detecting, clustering and weighting topics on the websites within a time period. Based on the idea of stock index, we also introduce a topic index approach in following the growth of topics, which is useful to analyze and forecast the development of topics on web.