To better understand the impact mechanism of online travel reviews, this study aims to investigate how the knowledge association of online travel reviews influences the sales volume of online tourism products from the perspective of knowledge-based theory and social network theory. We crawled online reviews of 314 tourism products from the leading OTA of China and constructed knowledge networks of online travel reviews by using the semantic association of thematic words. Our results show that both knowledge association and knowledge heterogeneity have a U-shaped curvilinear relationship with sales volume. Moreover, knowledge heterogeneity partially mediates the relationship between knowledge association and sales volume. In addition, knowledge cohesion has a negative moderating effect on the relationship between knowledge association and knowledge heterogeneity. The findings of our study have important theoretical value and managerial implications for tourism e-commerce research