To meet the application requirements of multi-antenna beamforming in 5G mobile communications, this paper designs and implements a multi-channel Software Defined Radio (SDR) communication system, and based on this system, achieves broadband multi-channel multi-frequency point synchronization calibration. To address the issue of multi-channel synchronization drift due to environmental factors such as temperature, this paper innovatively proposes a real-time synchronization scheme based on out-of-band calibration signals. This scheme inserts calibration signals into the redundant bandwidth of OFDM communications to track the drift of multi-channel response errors over time and compensates for the in-band effective signals. At the 1GHz frequency point, the system effectively compensates for the phase response error drift of 2.8 degrees and amplitude response error drift of 0.2dB caused by temperature variations through real-time calibration. After initialization calibration and real-time calibration, the phase error is controlled within 0.4 degrees, and the amplitude response error is controlled within 0.05dB in the temperature range of 40 to 80 degrees. This scheme not only achieves higher precision but is also completely transparent to the end user, allowing calibration without interrupting communication operations, making it highly significant for the design of 5G multi-channel synchronization.