Based on the three-plane waveguide theory, a simplified fast axis divergence angle(FADA) model of three bars laser diode arrays(LDA) is presented. Furthermore, on the basis of the model,the main influencing factors on FADA are as following: waveguide structure of single radiant cell including thickness of active layer and refractive index, wavelength and space between bars. The responses of FADA to these factors are analyzed by simulations. The establishment of FADA model of LDA provides theory supports for the research and application of LDA farfield characteristics.