In an effort to improve thermal stability and mechanical properties of porous polypropylene (PP) separators for lithium-ion battery, SiO2/PP/SiO2 composite separators were prepared by introducing SiO2 layer on both sides of PP separator through a dip-coating process, with polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) as binder. SiO2 nanoparticles are evenly distributed and closely packed in the coated layer, which features a porous honeycomb structure. This unique porous structure was quantitatively analyzed by Gurley value, and it can retain liquid electrolyte, leading to higher electrolyte uptake and ionic conductivity of the composite separator. The introduction of SiO2-coated layers can not only suppress thermal shrinkage but also improve mechanical properties of the composite separator. C-rate capability and cycle performance of composite separator were also investigated, and compared to those of pristine PP separator.