By virtue of the blockchain technology, smart contracts in transactions are presentable in the form of code, thereby fulfilling the contract in an automatized and irreversible approach, transcending the traditional contract model. The theory of self-help act and the theory of agency, which are deemed as an external reinforcement to the course of the contract's execution, are only a side explanation in a legal term that fails to grasp the essence. A smart contract does not necessarily cover the entire process of a contract, and instead, it perhaps simply constitutes an aspect or a stage of the contract. The smart contract in legal sense is not a new contract type, but a new development of contractual relationship, i.e., a new form to close and fulfil a contract by the parties concerned, whereas the nature of contract remains unchanged. Smart contract meeting specific requirements will constitute a special written form, which has the characteristics of "written form + automatic performance". When the civil subjects sign a contract by means of a smart contract, they should abide by the Civil Code and the relevant existing laws to ensure the legitimate establishment and validity of the contract. A smart contract is inequivalent to a contract with legal force, so the validity of it should be analyzed in specific scenarios during its application. For the sake of a long-term development of the smart contract, we should establish it in legislation, as well as compile and apply it in compliance with the current laws and policies to legalize the form and standardize the content of it. During the startup age of the technology, paper contract, general electronic contract and smart contract are functioning in tandem with one another in varying scenarios. This will become a principal status for civil contact and business transaction in the future.