Molecular dynamics simulations have been used to study the effect of the pre-orientation on the microstructure of lamella crystal and the stress response of polyvinyl alcohol (PVA) semicrystalline polymer under stretching. For the different pre-oriented systems, nucleation is demonstrated to be a two-step process, however, in a different intermediate order. For the isotropic PVA polymer melt, the segment needs more time to adjust its inter-chain structure, therefore, the nucleation is assisted by local order structures, while the nucleation of the oriented PVA melt is promoted by density fluctuation. The nucleation process is the result of coupling effect of conformational and orientational ordering. The transformation from flexible chains into conformational ordered segments circumvents the entropic penalty under the shear flow, which is the most peculiar and rate-limited step in polymer crystallization. Therefore, the current work suggests that the acceleration of the nucleation rate by shear deformation is mainly attributed to the different kinetic pathway via conformational/orientational ordering-density fluctuation-nucleation. From the different pre-oriented PVA semicrystalline polymers, we know that the higher oriented degree corresponds to a higher number of Tie chains and lower Loop chains, and the higher number of Tie chains corresponds to a stronger stress-strain response. And the detailed molecular structural evolution of semicrystalline polymer under stretching is also given in this work.