Fiber reinforced polypropylene (PP) is essential to the national development of lightweight equipment. The interfacial strength, as an important point, greatly affects its practical application effectiveness. In this piece of work, the realizing and controlling technology of interfacial crystallization was employed to strength the interface of polyamide 66 (PA66) fiber/PP system. The aramid β nucleating agent TMB-5 was added into PA66 fiber via molten mixing and melt spinning technology, as a consequence, the heterogeneous nucleation points were incorporated with PA66 fiber and induced the PP growing on the fiber surface to form interface crystals. Moreover, the experimental results showed that TMB-5 could orderly migrate from the PA66 fiber into PP matrix during heating, and hence generated a concentration gradient at the fiber surrounding. By adjusting the maximum heating temperature and concentration gradient of TMB-5 in the interface and nearby region, the TMB-5 was found to self-assemble into fibrous and dendritic like crystalline templates at the interface. Consequently, the β modified branched interface crystals embedded in the PP matrix were induced growing from the fiber surface. Compared to the conventional interface crystals, such as transcrystal, calabash et.al, the as-obtained interfacial structure owned both the advantages of multiscale fiber reinforcement and β form crystals, and exhibited great potential to enhance the interfacial strength between fiber and PP matrix.