A radio signal transmission method is used in a radio signal transmission system having a core network (CN), at least one base station and at least one user equipment (UE). After the UE initiates a circuit switch (CS) call, the base station determines whether to activate CS off-loading. If to activate CS off-loading, the base station determines whether identification (ID) codes of all call parties reside on the same base station or on the same group of base stations of the system. If yes, the base station determines whether to off-load only the user-plane data. If yes, the CN controls the signaling and the user-plane data is directly forwarded to another call party; otherwise, the base station processes a signaling and the user-plane data is directly forwarded to another call party via the base station.