Here we report highly efficient, indium tin oxide (ITO)-free polymer solar cells (PSCs) with an ultrathin copper (Cu) film(~10 nm) coated with a thin layer of poly[(9,9-bis(3‘-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) as transparent electrode. Despite of its lower far-field transmittance of the electrode, the obtained ITO-free device based on the ultrathin Cu film can delivery higher absorption efficiency than that of the device based on ITO substrate in the long wavelength region, which can be attributed to the formation of metal resonant microcavity between the opaque back metal mirror (MoO3/Al electrode) and the transparent Cu film with high reflectance. As a result, polymer solar cells based on poly[[2,6'-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-Th) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) blend show a high power conversion efficiency (PCE) of 8.21 %, comparable to that of the control device based on ITO electrode (with a PCE of 9.60% ). The results demonstrate that thermally evaporated Cu thin film electrode can be promising candidate to replace ITO for highly efficient PSCs, thus may open up the possibility for massive production of PSCs with low cost.