The mechanism of the orientational optical nonlinearity enhancement in the azo-dye doped liquid crystals is studied. The expressions of the enhancement factors based on the multiple-level transition model of azo-compound are derived. Our results show that, the simpler the model, the fewer the number of items contained in the expression of the enhancement factor. Using the enhancement factor expression obtained by the four-level model to describe the influences of the photoisomerization of azo-dyes on the enhancement of orientational optical nonlinearity is better than using that acquired by the three-level model. Besides, the expression of the enhancement factors in the case that the cis molecules do not absorb light is also obtained from the enhancement factor expression obtained by the four-level model. Except for a minor difference, the enhancement factor expression obtained in this case is the same as what was obtained for the anthraquinone-dye doped liquid crystals by Marrucci et al in 1997. This result also shows that under a given simplified condition, our theory is consistent with the last theory.