Among different structural types, modular buildings consist of a variety of modular units that are fully fabricated in the industry and transported to the site for installation. Due to their efficient construction and high product quality, steel modular buildings have become a current research hotspot. Due to the unique nature of steel modular buildings, it is important to consider the different design stages for this structure type. However, the information involved between different stages is complex, and at the same time, how to obtain high-performance designs under complex conditions is a challenging task. In order to address this issue, an integrated and collaborative optimization design method is proposed in the current paper. In the proposed method, the types of modular units that form the entire building and the structural components that constitute each unit are optimized. Through a case study of steel modular buildings, the effectiveness of the proposed method is validated and the designs with high performances are obtained.