The expressions of average refractive index in a helix pitch of ferroelectric liquid crystal（FLC） were derived by theoretical approximation computation of the helix structure at chiral smectic C phase of FLC. A conclusion that FLC molecules in a helix pitch as a whole can be considered as a model of nematic liquid crystal in terms of the expressions of average refractive index was obtained. When the thickness of FLC with a vertical alignment along the helical axis is integer times long as the helix pitch, FLC molecules can be considered as a group composed of nematic liquid crystal models. The comparisons of experiment results of ZLI-3654 FLC with 5CB nematic liquid crystal confirmed the above conclusion. The theory and experiment are in good agreement. Theoretical guidance and a deeper understanding can be provided by this theory for preparation of deformed helix FLC and vertical alignment deformed helix FLC devices, as well as application of ferroelectric liquid crystal.