Based on the theory of q-parameter, the collimating factor of the Gaussian beam transformed by the inverted telescope system has been strictly deduced. Assuming the aperture of lens is larger than beam size, the collimation factor of the system is determined by the geometric compression only, instead of waist radius or object distance. Further research found that under the appropriate approximation premise, the derivation results in this paper can be unified with the relevant derivations in the domestic laser principle textbook. The proposed algorithm is simulated, and the results show that the collimation factor obtained in this paper is consistent with the results obtained under the conditions discussed in the laser principle textbook. The collimation factor of the Gaussian beam through the inverted telephoto system obtained in this paper can provide a more accurate reference for the collimation operation part of related scientific research and engineering practice activities.