The rapid proliferation of multiple terminals in online communities has significantly transformed user behavior, making the effective management of these terminals crucial for user acquisition, retention, activation, and conversion. Despite the growing importance of multi-terminal environments, existing research has largely overlooked the interaction effects among terminals. To address this gap, this study investigates how multiple terminals interact in an online channel, specifically focusing on user transfer behavior between PC terminals and mobile terminals in an online Question-and-Answer (Q&A) community. Grounded in brand extension and expectation confirmation theories, we propose a theoretical model to examine the interaction effects among multiple terminals in Zhihu, a representative online Q&A community in China. We use SEM to analyze the data. Findings indicate that both substitution and synergy effects exist among multiple terminals, with the synergy effect being significantly stronger than the substitution effect. Furthermore, the results reveal that perceived quality, perceptual fit, expectation confirmation, and relative advantages affect the interaction among multiple terminals. Specifically, perceived quality and perceptual fit play significant roles in generating the synergy effect, while expectation confirmation and relative advantages drive the substitution effect. This study proposes a comprehensive framework for understanding multi-terminal interactions. The model holds both theoretical and practical relevance, as it helps in understanding user behavior in multiple terminal environments and provides implications for the design of multiple terminal strategies.