Abstract: A new model reduction method for dynamic analysis of complex structures based on Quasi-Rigid-Body Mode and Flexibility Modification is presented. The complex structures can be divided into several displacement synchronous parts by analyzing the finite element models based on the synchronistic assumption. Every synchronous part can be substituted by a quasi-rigid-body mode that is considered the extremum of the synchronous assumption, with which the full-size models can be reduced. According to the error analysis of the reduction process, the reduction model is updated by optimizing the transformation matrix based on flexibility modification and an accurate reduction model is obtained. As an example, an offshore jacket platform is presented to show the efficiency of the proposed method. And the result of the reduction model satisfies the precision requirement.