Amorphization by severe plastic deformation has been observed in various crystalline materials. However, developing a quantitative and comprehensive theory for strain-induced amorphization remains challenging due to the complex nature of microstructural evolutions and deformation mechanisms. We propose a phase field model coupled with elastic-plastic theory to study the strain-induced amorphization in nanocrystalline materials. The plastic behaviors of crystalline phases and amorphous phases are coupled with phase evolutions by finite strain theory through the strain energy. This coupled model enables to quantitatively explore the effects of various defects on formation of amorphous phases such as shear bands. Simulations using our model predict that amorphous nucleation follows the martensitic transformation, and occurs primarily within localized regions of high stress, including shear bands. Our results also indicate an increase in the critical plastic strain for amorphization as the grain size increases. These findings align well with experimental results, validating the proposed model in capturing key features of deformation-induced amorphization. Our work provides valuable insights into deformation-induced amorphization and serves as a basis for developing more quantitative models with complex microscopic mechanisms.