As scalability and flexibility have become the critical concerns in information management systems, self-organizing networks attract attentions from both research and industrial communities. This work proposes a semi-structured semantic overlay for information retrieval in large-scale selforganizing networks. With the autonomy to their own resources, the nodes are organized into a semantic overlay hosting topically discriminative communities. For information retrieval within a community, unstructured routing approach is employed for the sake of flexibility; While for joining new nodes and routing queries to a distant community, a structured mechanism is designed to save the traffic and time cost. Different from the semantic overlay in the literature, our proposal has three contributions: 1. we design topicbased indexing to form and maintain the semantic overlay, to guarantee both scalability and efficiency; 2. We introduce unstructured routing approach within the community, to allow flexible node joining and leaving; 3. We take advantage of the interaction among nodes to capture the overlay changes and make corresponding adaption in topic-based indexing.