Abstract: This invention discloses a method for switching SO (Service Option) to circuit data, including the following steps: step 1, a mobile station notifies a source BS (Base Station) and/or a target BS to switch SO to circuit data during a session; step 2, the source BS and/or the target BS negotiates with a mobile switching subsystem about switching SO to circuit data, and determines the bearer format parameters of the session through the negotiation; step 3, the source BS and/or the target BS makes a secondary service negotiation with the mobile station after the negotiation with the mobile switching subsystem succeeds; and step 4, after the negotiation with the mobile station succeeds, the source BS and/or the target BS conducts user interface service layer encapsulation on the circuit data according to a certain load format, and then transmits the encapsulated Real Time Transfer Protocol packets to the other BS.