The polypropylene (PP) separator is a kind of transversely isotropic porous polymer film, and it is a key component of lithium-ion batteries. The mechanical properties of the separator affect the strength and security of lithium-ion batteries directly. However, the anisotropy behaviors of the separator remain unclear, which has led to inaccuracy of failure behaviors in lithium-ion battery. A large deformation elastic–plastic constitutive model of the PP separator was developed with the Rich–Hill large deformation elastoplastic constitutive theory. Besides, the hardening law of the PP separator was established according to the Hill yield criterion. The constitutive model accurately captured the anisotropy behaviors and the elastic–plastic process considering the large deformation of the separator. Numerical examples for model validation were presented and in good agreement with stress–strain data of tests up to the hardening stage.