Compared with traditional closed innovation, open innovation brings more new ideas, technologies, and resources to megaprojects, which is becoming increasingly important in the face of growing complexity and sustainability. Drawing on the application of sense-making theory in organizational change, this study conducts an empirical case study to explore how open innovation is carried out in megaprojects, as well as how to explain the processes at the micro level. The Changchun Metropolitan Circle Ring Expressway phase II project was selected as the analytical case and grounded coding technology was adopted to analyze the realization process of open innovation. The results revealed that managers as organizers first completed their own sense making and then influenced others’ sense making via sense giving to unify understanding and complete innovation mode transformation. Moreover, managers need to adjust sense making for themselves and sense giving for others according to interactive feedback. This study illustrated that open innovation strategies need to enact some new relevant routines in order for a cognitive reorientation to take place. A multi-level and multi-dimensional micro-mechanism and process was proposed to guide open innovation behavior in megaprojects.