legal ground
Article 135 of the Civil Code of People's Republic of China (PRC)
A civil juristic act may take written form, oral form or other forms; Where laws and administrative regulations stipulate or the parties agree to adopt a specific form, it shall adopt a specific form.
Article 136
A civil juristic act shall take effect upon its establishment, except as otherwise provided by law or agreed by the parties.
Without the legal provisions or the consent of the other party, the actor may not change or terminate the civil juristic act.