The superior mechanical performance of natural rubber (NR) has been thought to originate from its strain induced crystallization (SIC) behavior, and molecular dynamics builds the bridge between microcosmic structure and macroscopic property, so a study on how SIC behavior affects chain dynamics is important to further understand how SIC behavior affect its mechanical property. In this paper, the SIC behaviors of unvulcanized natural rubber were studied by in situ synchrotron wideangle xray diffraction and the dielectric relaxation behaviors at different strain ratios by broadband dielectric relaxation spectroscopy. Through the research on the two dielectric relaxation modes of NR: segmental mode (SM) and normal mode (NM), we can get a deeper understand on the relationships between the evolution of chain conformation and mechanical performance. The results demonstrate that when stretched at the strain rate of 0.01 s^-1, unvulcanized natural rubber begins to SIC at the strain ratio about 3.0, and its dielectric relaxation behavior changes before and after SIC. Before SIC, the molecular chains orient with increasing strain ratio, so the dielectric strength Δε and relaxation time τ_HN of SM increase and decrease respectively. Meanwhile, the ideal glass transition temperature T₀ of SM and NM both increase. After SIC, the crystallinity increases with strain ratio and the crystals restrict the chain dynamics of NR, thus both for SM and NM, the dielectric strength Δε and relaxation time τ_HN decrease and increase with strain ratio, respectively.