We systematically researched flow-induced fibrillation at pressure (50 MPa) by using the custom-designed pressurizing and shearing device (PSD) equipped with well-designed dynamic sealing structure, which successfully solved the problem of melt leaking. Polarized Fourier transform infrared spectroscopy, small angle x-ray scatter and scanning electron microscope were employed to reveal the orientation of molecules and crystalline structure. The results clearly show that specific work of shear flow (extended to the case in the presence of pressure, which is closer to the industrial melt spinning) determines the degree of orientation (fc) or the formation of flow-induced fibrillation in isotactic polypropylene (iPP). Specific work of shear flow displays a critical value for the shear-induced fibrillation, suggesting a minimal energy needed for fiber formation in iPP melt, that is, melt cannot be induced to form fibers when the practical flow work imposed on melt is lower than the critical value. Additionally, even though higher pressure is beneficial for products to gain better performance, engineers should keep in mind that selecting a suitable processing pressure is crucial because the enhancement of pressure will make the situation tougher for flow-induced fibrillation and increase energy cost as well. These interesting and significant results shape a clear understanding in flow-induced fibrillation under pressure and provide the chance to lower the energy cost by optimizing processing conditions in synthetic fiber industry.