It just depends.
 1、 Only one party signs or seals the agreement. If the other party performs the main obligations of the agreement and is accepted by the other party, the agreement is valid;
 2、 If the above conditions are not met, the agreement is invalid.
 According to the law, a contract is concluded in the form of a letter of contract. If one party has performed its main obligations and the other party has accepted it before signing or sealing, the contract is formed.
 [Legal Basis]
 Article 490 of the Civil Code Where the parties enter into a contract in the form of a contract, the contract is formed when all the parties sign, seal or fingerprint. The contract is formed when one party has performed its main obligations and the other party has accepted them before signing, sealing or fingerprinting.
 A contract shall be concluded in writing as required by laws and administrative regulations or as agreed by the parties. A contract is formed when the parties fail to adopt a written form but one party has performed its main obligations and the other party has accepted it.