The present invention provides a method for realizing compatibility between a WCDMA system and a GSM system, including: in the downlink data path of a base station, In-phase/Quadrature (IQ) data of the GSM system are mapped to the Common Public Radio Interface (CPRI) according to a predetermined rate before IQ data interleaving and framing the CPRI Base Frame (BF), and then the mapped data of the GSM system are de-multiplexed with IQ data of the WCDMA system; in the uplink data path of the base station, IQ data in the CPRI are de-multiplexed after de-framing and IQ data de-interleaving CPRI BFs, and then the de-multiplexed IQ data of the GSM system are de-mapped according to the predetermined rate. The present invention further provides an apparatus for realizing compatibility between a WCDMA system and a GSM system. The method and apparatus genuinely realize the compatibility between the WCDMA system and the GSM system by unifying two different radio transmission modes between the WCDMA system and the GSM system in the bottom layer.