In recent years, blockchain technology 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. However, as the number of users and access requirements rise, the performance bottleneck induced by the poor scalability of the existing blockchain architectures has restricted the application and promotion of blockchain technology. How to solve the scalability problem has become a hotspot issue in academia and industry. This study analyzes and summarizes the currently available blockchain scaling solutions. For this purpose, the study outlines the basic concept of blockchain and the origin of the scalability problem, defines the scalability problem, and proposes the metrics for scalability. Then, it presents a classification framework and reports the existing solutions in the manner of categorizing them into three classes: network scaling, on-chain scaling, and off-chain scaling. Different blockchain scalability solutions are analyzed for a comparison of their respective technical characteristics and a summary of their advantages and disadvantages. Finally, this study discusses the open issues that need to be addressed promptly and explores the future trends of blockchain technology.