The Helfand's lattice theory of polymer interface has been extended to deal with the interface between thermotropic small molecular liquid crystals and flexible polymers. The main findings of the theoretical calculations are: (1) the interface of flexible polymer and the ordered nematic liquid crystal is very narrow due to the fact that the ordered nematic phase excludes strongly the flexible polymer segments; (2) the interfacial tension and the anisotropic factors of polymer bonds posses a first order transition, when the liquid crystal-rich phase undergoes the isotropic to nematic phase transition, which is characterized mainly by the fact that polymer bonds prefer to return back to the saturated polymer solution phase by reducing the probability of stretching into the liquid crystalline phase.