Abstract: In order to control lasing transverse mode, improve laser mode stability, and enhance laser processing quality, the transverse modes of a high power transverse-flow CO2 laser are measured experimentally, and the mechanism of formation of laser transverse modes is analyzed theoretically. The lasing transverse energy distribution of a high power tube-plate transverse-flow CO2 laser is simulated numerically by solving the space-time resolving rate equations derived from the discharging field determined by Maxwells equations. The result shows that the simulated lasing distribution agrees with the experimental observation. Moreover, there is evidence to show that the electrode configuration in the CO2 laser resonator determines the peak value of lasing transverse modes, while the gas flowing rate determines the lasing gain peak and the peak position along the gas flowing direction.