The exploration of inexpensive and efficient anode buffer layers is essential in large scale commercial applications of polymer solar cells (PSCs). Here, we report a simple way that can significantly enhance the power conversion efficiency (PCE) and extend the lifetime of PSCs. A solution-based tungsten oxide (WO3) layer with surface oxygen vacancies (V(O)s) is introduced as an efficient anode buffer layer between the active layer and indium tin oxide (ITO) glass. The PCEs of PSCs based on P3HT: PC61BM and PBDTTT-C: PC71BM active layers are improved by 24% (from 3.84% to 4.76%) and 27% (from 5.91% to 7.50%) with the introduction of the WO3 (V-O) anode buffer layer, respectively, compared to that of the conventional PEDOT: PSS layer. The excellent performance is ascribed to the greatly improved fill factor and enhanced short circuit current density of the devices, which are benefited from the surface with lots of VOs for better interfacial contact and excellent charge transport properties of the WO3(VO) layer. The impressive PCE, good stability, easy fabrication and compatibility with solution processed organic photovoltaic devices support this material's potential applications in PSCs for both wide bandgap and narrow bandgap polymers.