The output wavelength would change according to the change of the temperature of the gas mixture in a pulsed TEA CO2 laser, which is called the wavelength-temperature shift. A theoretical model based on the six-temperature mode rate equations is suggested to interpret such a phenomenon, which is used to calculate the wavelength-temperature shift relation of a TEA CO2 laser with plano-concave stable resonator. The numerical results indicate that the laser simultaneously generates several transition lines at one laser pulse, and the wavelength keeps positive change with the change of the ambient temperature. In addition, the laser′s wavelength-temperature shift occurs merely within the 10P branch no matter what ranges the temperature varies. So a grating resonator can easily eliminate the shift and keep the laser output spectra stable.