A new method of the finite element cavity surface segmentation was presented to simulate the propagation of light in the cubic cavity, and the linear formula between the average reflex optical path length(Lave) and cavity length of closed cubic cavity was calculated with MATLAB. At the same time, the Lave of cubic cavities with different cavity lengths were obtained by Tunable Diode Lave Absorption Spectroscopy(TDLAS) technique. Results show that the Lave will increase linearly with the increase of cavity length. Compared with the previous theoretical values, the experimental values have a better agreement with the simulated values of the finite element cavity surface segmentation and the relative errors between simulated values are no more than 2.5%. The linear formula between the Lave and cavity length can be used to decide the cavity length and enhances the inversion accuracy of TDLAS gas measurement system based on cubic cavity.