The atmospheric thermal lens in flying optics induced by high power CO2 laser during laser materials processing is studied. It is found in experiments that when the air in flying optics is static, atmospheric thermal lens can be induced. However, when the air is filled to the flying optics at certain pressure, atmospheric thermal lens can be eliminated. When there is no thermal lens, laser beam propagation obey the Gaussian theory; when there has thermal lens, laser beam propagation will deviate the Gaussian theory. This will have influence on laser focus size and position. And This deviation is calculated according to the nonlinear mode of laser induced thermal lens. The calculation coincides with the measurement. On the basis of it, two new conceptspseudo-beam waist and virtual-beam waist are proposed. And the paper points out that virtual-beam waist can be used to calculate laser beam propagation but not pseudo-beam waist.