This study examines how the external knowledge affects innovation. We categorize firms’ external knowledge sources into two modes: STI(science-technology-innovation) and DUI(doing-using-interacting). At the same time, we distinguish between the breadth and depth of firms’ external knowledge acquisition strategies, while considering the interaction between the knowledge source and the acquisition strategy. Previous studies have considered the source and the acquisition strategy of external knowledge independently. Thus, in order to fill this gap in the literature, we identify two types of external knowledge, each with its own searching mode, suggesting a 2 × 2 acquisition strategy model of external knowledge.