The flexoelectric effect has an influence on the distribution of liquid crystal in the strong anchoring hybrid-aligned nematic (HAN) liquid crystal cell, which can be measured by the improved full-leaky guided mode technique of liquid crystal. Based on the multi-layer optical theory and the elastic theory of liquid crystal, the variation curves between the reflectivity and transmittance of the full-leaky liquid crystal waveguide due to the strong anchoring hybrid-aligned nematic liquid crystal and the internal angle (the angle of incidence for which light is incident on the liquid crystal layer) are calculated theoretically, by which the influence of flexoelectric effect is analyzed. For different sign of the flexoelectric coefficient such as negative or positive, the variation curves of reflectivity or transmittance of the full-leaky liquid crystal waveguide will move a distance to the left or the right relative to the case of ignoring the flexoelectric effect, and the moved distance will vary with the different flexoelectric coefficients. The flexoelectric coefficients can be known from the moved distance, which provides the theoretical basis for determining the flexoelectric coefficients.