To meet the real time requirement of cost evaluating for construction project(CE/FCP), this paper proposes an effective evaluating method based on CBR (Case-based reasoning). The frame representation method of CE/FCP case is studied. To speed up case retrieval, decision tree algorithm is introduced to classify the cases, then the case adaptation and revising, case evaluation, case maintenance are introduced in detail. Finally, the method is applied into the CE/FCP practical problem, and the result obtained by the method is satisfied. It shows that the method not only can obtain solution very quickly, but also has strong adaptability. It is fitful to solve different CE/FCP problems when some basic evaluating data is lack.