In order to research the influence on the beam transmission properties due to the different time intervals in the high-power pulsed transversely excited atmospheric CO2 laser with unstable resonator, the finite element analysis of thermodynamics instantaneous method are adopted to analyze the mirror thermal deformation irradiated by the high-power laser beam. The mirror thermal deformation is fitted by Zernike polynomials. Then the angular spectrum propagation theory of diffraction is used to calculate the far-field transmission properties. The simulation results show that with the decrease in the time interval between each pulse, the mirror temperature and thermal deformation gradually increase, and peak power and the average energy density decrease, and beam broadens. With the 500 Hz repetition rate relative to the 10 Hz repetition, the peak intensity decreases almost 40%; the optical spot broadens about 60%. When the repetition rate is larger than 100 Hz, the surface of mirror will have obvious deformation, which will cause apparent degradation in the optical beam quality for the far-field transmission.