Social Q&A refers to the process of information seeking based on questioning-and-answering in natural language through social networks. It facilitates knowledge gathering and interacting, and thus can be regarded as an appropriate environment for informal learning. This study intends to explore factors affecting continuance intention of using social Q&A communities for informal learning and meanwhile reveal the underlying mechanism among these factors. Data were obtained from 257 university students through an online survey. By using partial least squares based structural equation model, the proposed hypotheses and research model were analyzed. The findings show that homophily, interactivity and social presence are three determinants of collaborative learning and community identification in social Q&A community which further influences university students' continuance intention for informal learning. Our research extends the understanding of informal learning in social Q&A communities and indicates how to promote continuous informal learning in such environment.