Method, device and system for establishing video session

The invention discloses a method, device and system for establishing a video session, belonging to the technical field of communication. The method comprises the following steps that: when a first mobile terminal issues a video session request to a second mobile terminal through an IM (Instant Messaging) server, a transit decision server receives an application for establishing a video session room, sent by the IM server, and networking types of the two mobile terminals, establishes the video session room, confirms the data transmission manner and a transit server and respectively sends the room information, the data transmission manner and the transit information to the two mobile terminals through the IM server; the transit server respectively receives the requests of entering the video session room, sent by the two mobile terminals, permits the two mobile terminals to enter the video session room and transmits the video information to the two mobile terminals after the information of the video session room