Legal analysis: As long as the contract elements are available, that is, the qualification of the parties, the determination of the subject matter, the relationship between rights and obligations, the signature and seal by fingerprints, and the expression of the will of both parties are valid, and the contract is legal and valid, not because one party is a prisoner. The reason why the contract is invalid is legal.

Legal basis: Article 490 of the Civil Code of People's Republic of China (PRC). If the parties enter into a contract in the form of a contract, the contract shall be established when the parties sign, seal or press their fingerprints. Before signing, sealing or fingerprinting, one party has fulfilled its main obligations, and the contract is established when the other party accepts it. A contract shall be concluded in written form as stipulated by laws, administrative regulations or agreed by the parties. If the parties do not do this in writing, but one party has fulfilled its main obligations and the other party accepts it, the contract is established.