Surface chemistry of the separator plays an important role in the performance of the lithium ion battery separator, which not only influence the wettability with the electrolyte, but also the lithium ion migration through the separator. Here we developed a simple method to modify the polypropylene separator with tannic acid (TA) and polyethyleneimine (PEI). A thin and uniform TA/PEI layer was formed onto the surfaces of the separator through a simple assembly process without destroyed the microstructure. The modified PP separator showed excellent wettability, high ambient ionic conductivity (0.95 mS cm−1) and lithium-ion transference number (0.44), indicating that the TA/PEI layer played a role in the lithium ions migration. The possible mechanism of the surface promoting lithium ion migration was discussed in this paper. The battery performances of the modified separator were also conducted. As a result, cells with the TA/PEI-coated PP separator displayed superior cycle stability and rate capability.