To fully unlock the potential of metallic electrodes in flexible polymer solar cells (PSCs), tuning their optical properties is urgently required. Here we report an efficient light harvesting scheme involving the combination of a silver mesowire grid-based front transparent electrode and a plasmonic meta-mirror-based back reflector electrode. As an alternative to the indium-tin-oxide transparent conductor, the silver mesowire grid on a plastic substrate enables the reduced ohmic loss with competitive mechanical, electrical and optical properties, verifying its superiority in large-area flexible devices. Further implementation of the plasmonic meta-mirror back reflector allows broadband enhancement of light harvesting efficiency over the entire visible wavelength range, yielding a power conversion efficiency of 9.50% due to a 23.2% increase in photocurrent compared to the reference flat device. Experimental and theoretical analysis of light propagation in PSCs elucidates that the optical harvesting enhancement benefits from the synergistic effects of wide-area redistribution of an optical field induced by the silver mesowire grid electrode and broadband recovery of photon loss via surface plasmon and scattering enabled by the meta-mirror back reflector. This method provides a convenient and scalable way for developing high-performance flexible PSCs towards the future photovoltaic applications.