Free-standing electrodes have been widely investigated for high performance flexible supercapacitors, especially those with polymer films as flexible substrates. However, the most widely used technique for the polymer-supported electrodes, in-situ solution polymerization, restricts their mass production. Here, a facile strategy was established for the mass production of flexible polymer-supported film electrode for high performance flexible symmetric solid-state supercapacitor via a two-step coating approach, with polylactic acid (PLA) as biodegradable and flexible substrate and polyaniline (PANI) as electroactive materials. Under the optimized condition, high specific capacitance of 615 F/g (areal capacitance of 143 mF/cm(2)) was achieved with the flexible film electrode at a current density of 1 A/g in 1.0M H2SO4. It possessed excellent cycling stability (109.6% of its original specific capacitance after 5000 CV cycles), flexible stability (105.6% capacitance retention after 180 degrees-bending 1000 times), and mechanical strength (tensile strength up to 16.89 MPa), owing to the inlay structure. The flexible symmetric solid-state supercapacitor (SSC) based on the composite film electrodes showed good comprehensive performance.