BlockchainDB: querable and immutable database. With the rise of a series of crypto-currencies, such as Bitcoin and Ether, the underlying blockchain technology has received more and more attention. The blockchain is known as the characteristics of decentralization and immutability. Ethereum utilizes the blockchain technology to build the next generation decentralized application platform. BigchainDB combines blockchain technology with traditional distributed databases, and uses the federal based voting to improve the traditional PoW mechanism and finally improves the system’s scalability and throughput. However, the existing blockchain system mostly stores transaction information with a fixed-form. Although there are data fields in each transaction, the existing blockchain system cannot directly query the specific details within the data fields of the transaction data from the blockchain data. To query the specific details of the data field, it must query the transaction first with the Hash value of the transaction to get the complete information of the transaction, and then retrieve the details in the transaction data. This mechanism has a low operability of data and a lack of query functions of the traditional database. This study first proposes a framework of blockchain database system, which applies blockchain technology to distributed data management. Then, an immutable index is proposed based on Hash functions. According to the index, the data in the block can be quickly retrieved to implement the query processing in the blockchain. Finally, experiments are designed to test the database’s read/write performance. The experimental results show that the immutable index has good read/write performance while ensuring immutability.