A polarization-insensitive plasmonic absorber is designed consisting of Au fishnet structures on a TiO₂ spacer/Ag mirror. The fishnet structures excite localized surface plasmon and generate hot electrons from the absorbed photons, while the TiO₂ layer induces Fabry–Perot resonance, and the Ag mirror acts as a back reflector. Through optimizing the TiO₂ layer thickness, numerical simulation shows that 97% of the incident light is absorbed in the Au layer. The maximum responsivity and external quantum efficiency of the device can approach 5 mA/W and ～1%, respectively, at the wavelength of 700 nm.