The Lagrangian finite element method (FEM) has evolved into a powerful numerical method and has been used in various engineering problems. However in extreme deformation problems, such as landslide, penetration and blast, mesh distortion seriously reduces the numerical accuracy and efficiency of the FEM. A novel finite element particle method (FEPM) is proposed in this work for modelling extreme deformation problems. In a material domain, regions undergoing small deformation are discretized using elements of the finite element method, while zones subject to extreme deformation are represented with particles of the material point method (MPM)[1,2]. A background grid, covering the entire computational domain, is employed to solve the momentum equations. To circumvent mesh distortion, distorted elements under extreme deformation are adaptively converted into particles during the simulation. The seamless coupling between elements and particles is naturally achieved through the background grid’s single-valued velocity field. Furthermore, a contact method is introduced to handle interactions between distinct material domains. Several numerical examples, including symmetric rod impact, soil collapse, soil collapse with a base, penetration of a plate by a long rod projectile, and penetration of a plate by an explosively formed projectile, are studied using the proposed FEPM. The numerical results exhibit good agreement with published literature data and experimental results, demonstrating the effectiveness of FEPM in simulating extreme deformation scenarios.