The invention provides a system, a method and a multiplexer for testing radio frequency for a base station. The multiplexer comprises a first port, a first combiner, at least a second port, at least a third port, a band-pass filter and a band elimination filter. The first port is connected with a radio frequency port to be tested of the base station, the first combiner is connected with the first port, the second port is connected with a frequency spectrum analyzer, and the third port is connected with a signal generator. The band-pass filter is connected between the first combiner and the second port, and a passing band of the band-pass filter is a downlink radio frequency signal frequency range of the base station. The band elimination filter is connected between the first combiner and the third port, and a stop band is a downlink radio frequency signal frequency range of the base station. The system, the method and the multiplexer can improve accuracy of testing efficiency and testing results and reduce testing cost.