The crystal structure evolution of polyethylene (PE) gel films containing paraffin oil are studied with wide-angle X-ray scattering (WAXS) during sequentially biaxial stretching at the temperature around the melting point. Also, the surface morphologies of films after biaxial stretching are characterized by scanning electron microscope (SEM). During machine direction (MD) stretching process, the crystal structure is transformed from lamellar stacks to fibrillar crystals through melting recrystallization. While for transverse direction (TD) stretching, there exist three kinds of deformation mechanisms determined by draw ratio in MD (lambda(MD)), i.e., transformation of weakly oriented crystals into fibrillar crystals by melting recrystallization at small lambda(MD); with moderate lambda(MD), the fibrillar crystals in MD are filled towards TD; at larger lambda(MD), the microfibrils are reared into thinner ones or fibrillar crystals, inclining to TD. Finally, the crystal morphology diagram is summarized in two-dimensional (2D) strain space, serving a roadmap for processing of PE microporous membranes.