Aerogel fibers with high porosity, low thermal conductivity and flexibility have shown great potential for applications in personal thermal management. However, the porous structure of aerogel fibers significantly compromises their mechanical properties like tensile strength. Here, we propose a high-strength polyimide aerogel fiber with porous-cortex-dense-core structure prepared via a coaxial wet-spinning based on hierarchical phase separation. Porous-cortex is formed due to fast phase separation rate induced by weak electrostatic and hydrogen-bonding interactions between the fluorinated polyimide and the ethanol. In contrast, the poly(amic acid) with high polarity index in the core-layer exhibits a slow phase separation rate, allowing the fibers to produce a dense nanoporous structure. With the dense core undertaking stress and porous cortex hindering heat transfer, the obtained aerogel fiber exhibits a higher tensile strength of up to 55.2 MPa compared to most reported aerogel fibers (0.15 –30 MPa) and a low thermal conductivity of 37.2 mW m-1 K-1. This work offers a new way to prepare strong aerogel fibers and broadens their applications in thermal protection and infrared stealth.