The accuracy of beam position measurement is affected by the gain inconsistency of the four channels of the digital BPM. Based on the existing hardware and the requirement of beam position monitor (BPM) measurement accuracy, a design for automatic calibration of digital BPM sample data gain is implemented by Verilog language in the self-made electronics. Firstly, the system design of automatic gain calibration module is introduced. Secondly, the realization method of the module is described in detail, and the ADC data automatic gain calibration test platform is designed and built to verify the function of the automatic gain calibration module. Finally, the application of this design in BPM channel calibration is introduced. Test results show that this method achieves 4-channel gain consistency and makes the data amplitude after ADC sampling equal. This method effectively solves the measurement bias caused by channel gain inconsistency and the difficulty of ADC data amplitude calibration in engineering application. It will play an important role in the automatic channel calibration of BPM system.