Effective improvement molecular orientation in the amorphous region and regulating crystallization transformation behavior is of great significance for the industrial production of ultra-strength polycaprolactam (HSPA6) fiber. In this work, the effects of adding a small quantity of high molecular weight polycaprolactam (HmwPA) on the crystal transformation, melt viscoelasticity, molecular orientation and mechanical properties of HSPA6 fibers were systematically investigated. The results indicated that HmwPA increased the elastic modulus and loss modulus of HSPA6 melt and suppressed molecular relaxation during drawing to form a crystal structure dominated by α-form. As such, the tensile strength and elongation at break of HSPA6 fiber reached 10.32 cN/dtex and 18.7%, respectively. 1D and 2D wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and birefringence observations demonstrated that the HmwPA not only inhibited the destruction of the crystal structure during fiber drawing but also increased the molecular orientation in the amorphous region from 63.1% to 70.1%, while maintaining the crystal orientation to about 95%.