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Unstructured peer-to-peer (P2P) systems with two-layer hierarchy, comprising an upper layer of super-peers and an underlying layer of ordinary peers, are used to improve the performance of large-scale P2P systems. In order to deal with continuous growth of participating peers, a scalable super-peer overlay topology with a lower diameter is essential. However, there is relatively little research conducted on constructing a scalable super-peer overlay topology. In the existing solutions, the number of connections that super-peers need to maintain is in direct proportion to the total number of super-peers which makes the solutions not scalable as well as not practical. Therefore, in this paper, we propose a scalable hierarchical unstructured P2P system which using a self-similar square network graph (SSNG) to construct and maintain the super-peer overlay topology dynamically. Moreover, a forwarding mechanism over SSNG is presented to enable each super-peer to receive just one flooding message. The analytical results show that the proposed SSNG-based overlay is more scalable and efficient than the perfect difference graph (PDG)-based overlay proposed in the literature.