The physical model of multiple beams interference fringes was established for unstable resonator alignment, and the characteristics of interference fringes with different misalignment angles were calculated by a homemade software, in which augmented matrices of misaligned optical systems was applied to compute the decentration and tilt of Gaussian beams. The ABCD law was applied to calculate the multi-pass propagation of Gaussian beam q parameter. Furthermore, the interference fringe for unstable resonator alignment was calculated by adding the complex electrical field of multiple Gaussian beams. The calculated interference fringe was found to fit well with the experimental results, which shows that the physical model is correct. Therefore, this model can quantitatively explain the relation between the characteristics of interference fringes and the misalignment of the resonator, and then can be a theoretical guidance for compute-aided automatic alignment of the resonators. 