Offchain channel network (OCN) can effectively improve the performance of blockchain system. The key component for OCN to achieve long-term efficient and stable operation is routing algorithm. This study proposes OCN architecture and the basic model of offchain channel routing algorithm. From perspectives of single-path routing and multi-path routing, typical routing algorithms are systematically reviewed and discussed. Meanwhile, an evaluation system is established for offchain channel routing algorithm, in terms of effectiveness, concurrency, scalability, channel balance, routing centralization, cost-effectiveness, privacy protection, goodput, latency, success rate, and efficiency. Finally, these algorithms are compared, and challenging research issues and technology trends of offchain routing algorithm are discussed.