A session routing method, an apparatus and a system are disclosed here. The method comprises that when the first terminal accesses the first network which the first routing apparatus belongs to, the address of the second routing apparatus in the second network is obtained by the first routing apparatus; the tunnel between the first routing apparatus and the second routing apparatus is established by the first routing apparatus according to the address of the second routing apparatus; and the session between the first terminal and the second routing apparatus is maintained through the tunnel by the first routing apparatus, so that the session is routed to the second terminal in the second network by the second routing apparatus. The invention doesn't need to set home network for the terminal, after the terminal accesses a certain network, the invention realizes the session routing between the terminal and the routing apparatus in the other network by the routing apparatus which the terminal attached in the network. Because the routing apparatus which the terminal belongs to is changed according to the different networks which the terminal belongs to, the communication flow of the terminal doesn't concentrate on one routing apparatus to be transferred, and the processing load of the single routing apparatus is reduced.