Blockchain is a new type of distributed computing paradigm and is de-trusted. It is a distributed ledger technology (DLT) based on game theory consensus. The emergence of blockchain promotes the transformation of information Internet to value Internet and accelerates the generation of programmable currency, programmable finance and programmable society. The blockchain is bound to have a revolutionary impact on the fields of finance, Internet of Things, credit reporting, etc. In terms of improving production efficiency, reducing production costs, and protecting data security, the blockchain will play an important role. The blockchain will have a positive impact on data security and network security. Meanwhile, the blockchain itself is facing serious security problems, which has attracted extensive attention from researchers. This paper introduces the basic principle of blockchain in modules. Focusing on the algorithm, protocol, application, implementation, and system perspectives, this paper overviews the security problems of the blockchain technology and discusses the essential rationale for them. It mainly analyzes the consensus algorithms in the security of the protocol, the smart contracts in security of implementation, and security issues in digital currency exchanges in practical applications. The defects of existing blockchain security protection measures are analyzed, and the problem of blockchain security is given. Finally, the defects of existing protection measures to blockchain security problems are analyzed, some ideas toward the solutions are given, and the future research directions are clarified.