In the field of information security risk evaluation, it is difficult for the traditional static evaluation methods to adapt to the dynamic topology of Mobile Ad hoc NETwork (MANET). In order to solve the problem, a new dynamic reevaluation model was proposed. In the proposed model, the whole system was abstracted into a topology which was comprised of components and access paths. The relations between components were abstracted into three kinds of association relations and four kinds of combination relations. In addition, the methods of security metrics under different relations were provided. When system changed, the influence range of its change and new relation types were determined by taking the changed component as a center. Under that condition, only adjacent components were reevaluated. Then, the new local and whole security metrics were obtained. The experimental results show that the proposed model has higher evaluation efficiency and can decrease evaluation cost greatly.