Based on the Gaussian beam propagation theory, and combined with the structure of the laser diode stack, the fast axis beam model of the laser diode stacks was established. And considered with the divergence angle and directional factor of each diode laser array, the calculation for the fast axis beam quality of the diode laser stacks was amended which was finally verified by the experiment. The result shows that compared with the experimental measurements, the calculation error of this model is only 2.14%. Compared with the ones before amending, which are 24.16% and 18.36%, the new calculation method is improved a lot in accuracy and can reflect the fast axis beam quality of diode laser stacks more exactitudly.