In recent years, blockchain has attracted a lot of attention. As a distributed ledger technology, it has been applied to many fields due to its openness, transparency, and non-tamperability. As the increase of the number of users and demand, the low processing speed of blockchain becomes a performance bottleneck. The poor scalability of the ex-isting blockchain architectures has restricted the application and promotion of blockchain. How to solve the scalability problem has become a hotspot of the blockchain field in academia and industry. This paper analyzes and summarizes the solutions on the scalability of blockchain. Firstly, we introduce the scalability problem. More broadly, we define the scalability problem and extract metrics to measure it. Secondly, we give a general classifica-tion framework, which categorizes existing solutions into three types: network expansion, on chain expansion, and off chain expansion. Through the analysis of different blockchain scalability schemes, the respective technical characteristics are compared and the advantages and disadvantages are summarized. Finally, we discuss open issues that need to be addressed, and explore future trends in blockchain.