Embodiments of the invention disclose a method and a device for synchronizing messages of multiple instances, which relate to the technical field of information and are capable of guaranteeing synchronization of the messages of online instances under a receiver user account while guaranteeing synchronization of the messages of online instances under a sender user account. The method comprises the following steps of firstly, receiving a message which is sent by a first instance and obtaining a second instance user account corresponding to the message; secondly, obtaining online instances of a first instance user account and online instances of the second instance user account; and lastly, sending updating and synchronizing notifications to the online instances of the first instance user account and the online instances of the second instance user account in order to carry out message synchronization on the online instances of the first instance user account and the online instances of the second instance user account.