Invalid. If the contract is in writing and only has the signature of Party A but not the signature and seal of Party B, the contract will be invalid. If the contract is in electronic form, a confirmation must be signed in advance, otherwise, the contract will be invalid. Since it is a contract, it needs to be signed by both parties. Article 490 of the Civil Code: If the parties conclude a contract in the form of a contract, the contract shall be established when both parties sign, seal or fingerprint the contract. Before signing, sealing or fingerprinting, one party has performed its main obligations and the contract is established when the other party accepts it. Laws and administrative regulations stipulate or the parties agree that a contract should be concluded in written form. If the parties do not use written form but one party has performed its main obligations and the other party accepts it, the contract is established.