As an advanced technology, blockchain is the core of trust establishment, data interaction, and value delivery in a distributed zero-trust environment. However, due to the characteristics of equal autonomy of nodes, multi-party maintenance of data, and closed isolation between chains, the blockchain system faces the problem of impossible triangle. Many researchers have studied blockchain scalability methods. This paper investigates the existing methods from three aspects of the network scalability, throughput scalability, and function scalability. It also compares and summarizes these methods, aiming to direct future research in the scalability optimization of blockchain.