In order to solve the problem of deviation in the calculation method of traditional lidar geometric factors， the full field and half field of view functions in the telescope are derived through geometric optics， and the geometric factor is obtained by the ratio of the double integral of the field of view function of the uniformly emitted laser beam area to the cross-sectional area of the laser beam. Considering the effect of the laser optical axis tilt misalignment， the geometric factor calculation equation for the atmospheric detection coaxial lidar is derived. According to the proposed calculation equation， the geometric factor of the typical atmospheric detection coaxial lidar is calculated and compared with the traditional method result. The result shows that the minimum full overlap distance is consistent. Finally， the effects of laser beam diameter， divergence angle， and laser optical axis tilt angle related to excitation-emission on geometric factors are analyzed. The results show that the influence of the laser divergence angle is greater than that of the beam diameter. The method is suitable for the rapid calculation of the lidar geometric factor of the uniformly distributed laser emission beam and can provide a reference for the design of the lidar.