All solution-processing at low temperatures is important and desirable for making printed photovoltaic devices and also offers a possibility of safe and cost-effective fabrication environment for the devices. Herein, we report an all solution-processed flexible organic solar cell (OSC) using poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) electrodes. The all solution-processed flexible devices yielded the highest power conversion efficiency of 10.12% with high fill factor of over 70%, which is the highest value for metal oxide-free flexible OSCs reported so far. The enhanced performance is attributed to the newly developed gentle acid treatment at room temperature that enables a high-performance PEDOT:PSS/plastic underlying substrate with the matched work function (≈4.91 eV), and the interface engineering that endows the devices with better interface contacts and improved hole mobility. Furthermore, the flexible devices exhibited an excellent mechanical flexibility, as indicated by a high retention (≈94%) of the initial efficiency after 1000 bending cycles. This work provides a simple route to fabricate high-performance all solution-processed flexible OSCs, which is important for the development of printing, blading and roll-to-roll technologies.