Considering the structural characteristics of 5 kW transverse flow continuous wave CO2 laser electrically excited with pin-to-flat discharge, the 90° conical reflector designed as the resonator cavity of totally reflective mirror is investigated. Based on the Fresnel-Kirchhoff theory, the diffraction integrals equation of conical resonator is deduced by using Collins formula and taking gain influence into account. The results indicate that the resonator composed of conical reflector and plane output mirror is stable and low-step output modes are acquired for high power transverse flow CO2 laser. Limited by the processing accuracy, there is a processing residual blind-hole in the center, which brings considerable loss for TEMmn (m=0, n≠0) modes, so its output is difficult. However, the low-order modes of weak center field distribution for less loss have prior output.