1. A seamless session switching method used in a VOIP session, comprising: a first mobile terminal establishing a first VOIP session with a second terminal via an initial wireless network; the first mobile terminal acquiring a target wireless network when the first mobile terminal detects that a switching event occurs; the first terminal transmitting a second session request to the second terminal via the target wireless network, and keeping the first VOIP session continued; and the first mobile terminal establishing a second VOIP session with the second terminal via the target wireless network after receiving an acknowledge message for the second session request from the second terminal, and terminating the first VOIP session after the second VOIP session is established.