In this paper the multi-task interaction scenario is analyzed and an activity-centered method to manage personal information is presented. An activity model is discussed from the aspects of the static structure and dynamic involvement of an activity and relation between activities. A method of computing the relativity between activities from the aspects of the interaction characteristic in multi-task scenario and the content of the object involving an activity is proposed. Moreover, a personal information management centered-activity tool ACPIM (activity-centered personal information management) is designed. An evaluation of this ACPIM system showed that the tool could reduce cognition overload for users and be useful in their own work.