Under certain conditions, polypeptide liquid crystal may exhibit an isotropic-anisotropic transition coupled with the intramolecular coil-helix changes. Based on the theoretical model that describes this phenomenon well [Lin J, et al. Polymer 2000; Lin J. Polymer 1997, 1998], the influence of external orientational held on such conformationally coupled transition was studied by introducing an energy term related to the quadrupole field. The calculations carried out show that the coupled isotropic-anisotropic and coil-helix transitions are accelerated when an external orientational field is applied. The effect of the external field intensity and molecular parameters of polypeptide has also been studied. The information obtained through the investigation may find important applications, such as envisaging new routes for processing the rigid-flexible block copolymer with better mechanical properties and constructing molecular devices for information storage and processing.