Reciprocity calibration is one of the major practical challenges for massive multi-input multi-output (MIMO) systems operating in Time-Division Duplexing (TDD) mode. A new reciprocity calibration method is studied in this paper which can increase the system performance and also support online calibration. Firstly, a test device (TD) is used to send pilots to the base station (BS) to obtain the uplink (UL) channel state information (CSI). Secondly, the BS sends pilots to the TD to get the downlink (DL) CSI. Thirdly, the DL CSI is transmitted to the BS via the Internet and the calibration coefficient is calculated. Finally, by multiplying the calibration coefficient with the UL CSI from the user equipment (UE), the reciprocity calibration is performed. A simulation based on the experimental measurement results shows that the performance of our proposed method is improved significantly compared to the existing relative calibration methods.