Inter-organizational collaborative innovation is crucial for advancing the development of prefabricated buildings (PBs). However, the structural characteristics and formation mechanisms of networks arising from such collaborative innovation remain unexplored. Using patent data on PBs in China (2010-2021), this study constructed patent collaboration networks (PCNs) and examined their structural characteristics using social network analysis and network motif analysis methods. Exponential random graph models were applied to investigate the mechanisms underlying PCN formation. The results show that (i) the PCNs of PBs exhibit small-world and scale-free characteristics without a core-periphery structure, evolving from single-core to multi-core aggregation and forming an open and diverse innovation ecosystem; (ii) State Grid Corporation of China emerges as the core organization, significantly influencing knowledge flow and resource integration; (iii) enterprise-enterprise-enterprise and enterprise-enterprise-university are dominant collaborative innovation patterns for PBs. Enterprises lead PBs' innovation while universities provide technical support; (iv) PCNs' formation is driven by triadic closure, technology proximity, geography proximity, collaboration breadth, and collaboration depth, whereas organization proximity and innovation capability show inconsistent effects. Robustness tests conducted on PCNs formed by varying time intervals also support these findings. These insights provide strategic guidance for partner selection in PBs' innovation and inform policy-making for collaborative innovation initiatives.