Abstract: There are many task oriented dialog systems, but few have taken into account the issues on multiple topics, topic shifts, information sharing among topics, and difference in importance for information items. A dialog management model based on the topic forest structure is presented here, which can well handle those issues with mixed-initiative dialog control. This model is consistent during the dialog transaction, so that it can understand the user according to the context. Its dialog control and response decision are both natural and flexible. With several domain-independent strategies, its reasoning engine is portable too.