In this paper, a solution to reengineering the legacy system based on mobile agent technology is proposed to satisfy the demand of continuous introducing new requirements and techniques into legacy system. An agent viewpoint is adopted to model the legacy system, with the components frequently interacting with others implemented as mobile agent and new requirements added as customized agents. Through the practice of migrating a standalone single-user computing software onto networking platform with the support of several remote user's concurrent accesses, an alternative way of introducing new requirements and technologies into legacy system is proposed.