Carrying out opportunistic collaboration, a method of flexible collaboration centering around ideas and free collaboration structures, is important in knowledge creation organizations, especially for knowledge-building community formation. However, fixed-group collaboration is still widely employed in educational practice, hindering the development of students' knowledge creation activities. In this design-based study, we created and applied distance-shortening strategies, which are strategies for shortening students' physical distance and idea distance, to support their opportunistic collaboration. The participants were 24 master's degree students who took a required one-semester course titled Learning Sciences in Knowledge-Building Environments that included online and offline activities. Data included (1) records of students' online activities; (2) video clips of students' offline activities; and (3) the content of students' online notes. Social network analysis, video analysis and content analysis were applied. The findings revealed that with distance-shortening strategies for constructing community knowledge and collective responsibility, the students were able to overcome the barriers of a fixed group and engage in opportunistic collaboration. Implications for principle-based and design-oriented knowledge-building activities and approaches to fostering knowledge creation are discussed.