We demonstrate a new kind of composite separators. A unique feature of the separators is the three-tier structure, i.e. the crosslinked polyethylene glycol (PEG) skin layer being formed on both sides of the nonwoven separators by in-situ polymerization and the large pores in the interior of the nonwoven separators being remained. The surface pore structure and the thickness of the skin layer could be adjusted by controlling the concentration of the coating solution. The skin layer is proved to be able to provide internal short circuit protection, to contribute a more stable interfacial resistance and to alleviate liquid electrolyte leakage effectively, yielding an excellent cyclability. The remained large pores in the interior of the composite separators could provide an access for the fast transportation of lithium ions, giving rise to a very high ion conductivity. The polyimide (PI) nonwoven is employed to ensure enhanced thermal stability of the composite separators. More notably, the composite separators fabricated from the coating solution with a composition ratio of 20 wt% provide superior cell performances owing to the well-tailored microporous structure, comparing with the commercialized polypropylene (PP) separator, which show great promise for the application in the high power lithium ion batteries.