Homogeneous mixtures undergo phase separation to generate rich heterogeneous structures as well as enable complex physiological activity and delicate design of artificial materials. Beyond free space, the strong coupling between migrating components and spatial confinement plays a crucial role in determining the essential spatial compartment of phase separation, warranting further continuous exploration. Herein, we report the selective phase separation (SPS) behavior of polymers under a mobile two-dimensional (2D) confinement by graphene oxide (GO) sheets. The selection of a poor solvent triggers the occurrence of SPS in a homogeneous solution of GO and polymers. We reveal that the self-limiting spatial confinement of GO sheets leads to the migration of polymers to form independent and continuous phase in 2D confinement. We examine the quantitative rule of size and continuity of polymer phases in correlation with solvent properties and solute constitutes. The observed SPS allows the facile generation of heterogenous nanostructures in GO/polymer composites. We initiate a SPS wet-spinning to fabricate radial heterogenous fibrous graphene composite fibers with ultrahigh elongation at break and superior flexibility. The observed SPS can inspire more exceptional phase separation behaviors under mobile 2D confinement and offers a facile method to delicately design 2D heterogeneous nanostructured materials.