Known as the core technology of bitcoin, blockchain is a distributed ledger technology, which is decentralizated, open, tamper-resistant, and autonomous. BaaS (blockchain as a service) takes blockchain as infrastructure and provides many services satisfying the user requirements. BaaS has become the focus of the cloud computing. This study discusses the architecture of BaaS and the preliminary design of each module, referring to the corresponding researches of BaaS and the current related enterprise projects. It is also analyzed that the characteristics of related technologies while integrating cloud computing with BaaS and the possible threat model is given. At last, the future prospects of BaaS are analyzed after investigating the current BaaS applications. In summary, this study aims to provide a full-knowledge and a reference architectural model of BaaS.