In order to study the formation and improvement of laser modes of high power transverse-flow CO2 laser,the structure of a tube-plate electrode is analyzed theoretically. A Laplace′s equation describing the electric potential in the discharge area is numerically solved,and the spatial distributions of the electric potential and field are obtained. Based on the distributions of electric field,the densities of electron,population and photon can be predicted theoretically. These results can be used to analyze the formation reasons of laser modes,as well as to explain the nonuniform characteristic of the laser spot energy. It is also shown by the calculations that with the decreasing of electrodes space and increasing of the tube diameter,the magnitudes and uniformity of electric field near the optic axis are enhanced greatly. As a result,the electron density will be increased and the laser modes can be controlled and improved. Based on the theoretical analysis,laser beam modes are improved through slightly adjusting of the electrodes and the optic axis.