Based on the study of key management schemes and blockchain technology, a hierarchical IoT key management scheme using blockchain is proposed. This scheme encapsulates the key generation process in the blockchain as a smart contract, creating communication keys only for adjacent nodes that communicate. When the contract receives a key generation request, it automatically generates the key, ensuring the process is tamper-proof and reliable. The generated public key is output as the transaction result, which terminal nodes can obtain by viewing. Security analysis and experiments show that this scheme reduces the communication overhead of key distribution, ensures the security of key transmission, and improves the reliability of key management.