With the rapid development of E-learning, collaborative learning is an important kind of group learning mode. It emphasizes the communication and information sharing among group members. Current collaborative learning systems haven't reached a high interaction degree between the students and the instructor to gain knowledge. In order to solve this problem, this paper introduces a semantic-based automated question answering (Q&A) system with a learning object repository like a virtual teacher to respond to student questions online. Through the proposed mechanism, the Q&A knowledge base will be enriched in the future process of Q&A. Furthermore, not only the students can get answers to their questions, but also the instructors could know what problems students encountered in learning. This would be a great help for both the instructor and the students in collaborative learning environment.