Massive multiple-input multiple-output (MIMO) technology, as a key technology in 5G mobile communications, greatly improves communication capacity, and is an important development direction for rail transit communications. In this paper, the channel measurement of the 32x32 massive MIMO system is carried out at 3.5 GHz and 5.6 GHz frequency bands in a real subway tunnel environment. According to the measurement data, the characteristics of the massive MIMO channel in subway tunnels are analyzed. It is found that the path loss index of the 5.6 GHz band is slightly lower than that of the 3.5 GHz band. By comparing the polarization crossing rate, it is found that the arc tunnel has a stronger depolarization characteristic for +45° polarization and horizontal polarization. The root mean square delay spread (RMS-DS) of the signal in the tunnel is generally distributed within 10 ns. In addition, it is found that the channel capacity gradually decreases with the increase of the transceiver distance. The 32x32 MIMO channel capacity at 3.5 GHz frequency band is 54.64 bit/s/Hz, which is slightly higher than the 53.35 bit/s/Hz at 5.6 GHz frequency band. And it is found that when the polarization mode of the antenna element matches the channel polarization characteristics or the number of elements increases, the channel capacity will increase. These findings can provide a basis for the application of massive MIMO technology in subway tunnels in the future.