Compared to traditional buildings, Modular integrated Construction (MiC) buildings are receiving increasingly attention due to their construction efficiency and high quality. With the fast development of MiC buildings, structural safety is an important topic, and investigations of their seismic performance is one of the most important aspects. In order to improve the seismic resistant capability of MiC structures, extra seismic devices were commonly adopted, such as rubber isolation and Tuned Mass Damper (TMD) devices. In the current paper, an analysis method is proposed to investigate the seismic performance of steel modular structures with isolations and TMDs. Through two case studies, the influence of these devices on modular structures is discussed, and the effectiveness of the proposed analysis method is validated.