A new generalization approach based on feature analysis method is proposed to deal with buildings with complex flat roof. This approach divides features of building roof into three types, which are two-edge feature, three edge feature and component feature. Simplification algorithms are developed to deal with different feature types. By analyzing the relationship of the three algorithms, a detailed process to simplify complex flat roof is developed. The approach is implemented and tested on a number of 3D buildings. Two typical buildings and a building cluster are simplified by the proposed algorithm under different threshold. The experiment results show that the 3D building can be efficiently generalized, while the characteristics of roof structure can be well preserved after simplification.