The channel measurement based on pseudo-random sequence was carried out for 5.6 GHz Multiple Input Multiple Output (MIMO) system in Shanghai metro tunnel environment. The impulse response of subchannel is obtained by correlation operation. The effects of antenna polarization and array element spacing on the condition number, equivalent degree of freedom and channel capacity of MIMO channels are analyzed. The results show that when the average number of cross polarization conditions of the antenna is about 10, it is about 5 and 8 lower than vertical polarization and horizontal co polarization respectively. When the mean value of cross polarization equivalent degree of freedom is about 16, it is about 03 and 02 higher than that of vertical polarization and horizontal homopolarization respectively. When the average cross polarization capacity is about 58 bit/s/Hz, it is about 07 bit/s/Hz and 08 bit/s/Hz higher than that of vertical polarization and horizontal polarization respectively. All three results agree that the cross polarization of antenna is beneficial to the improvement of MIMO performance. It is also found that the antenna array element spacing has a greater impact on the channel capacity of co polarized MIMO. The channel capacity of cross polarized MIMO is less affected. It shows that cross polarization can obtain larger channel capacity with smaller array element spacing. These results have reference significance for reducing antenna size and increasing channel capacity in narrow space of subway tunnel.