Polymer-based nanocomposites have exhibited the potential application prospects in spacecraft serving in low earth orbits (LEO) for preventing atomic oxygen (AO) erosion. However, the high-performance AO-resistance polymer nanocomposites remains a big challenge due to the lack of knowledge of the protection mechanism and the advanced composite technology. The theoretical calculation (DFT) results demonstrate that the defective graphene (DG) owns lower binding activation energy with AO compared with pure graphene and polymer matrix, which is beneficial to protect the polymer matrix from AO erosion as a nano-filler. Herein, a series alkylated DG (alkylated-reduced graphene oxide, RGO) with designed size were prepared and incorporated in thermoplastic polymer by melting blend, and the RGO endows the polymer matrix with obvious enhancement in terms of mechanical properties and AO resistance. Moreover, the liner structure–performance relationship of polypropylene/RGO composites was established after proposing a quantitative model of RGO aspect ratio and further fitting the enhancement quantity with RGO aspect ratio.