As a zero-bandgap and single-layer material, graphene has outstanding optical properties and electrical conductivity. However, graphene shows a poor ability to absorb light in the mid-infrared region. It can be enhanced by patterning metal on graphene into periodic metamaterials, and the photoconductivity can be tailored by fermi-level tuning. Here, we demonstrate polarization-sensitive and filter-sensitive photodetection in a tri-layer graphene structure. The experiments show that the thermoelectric and photovoltaic effects competed under different backgate voltages. Plasmon–phonon quasi-particle decay into electron-hole pairs and phonons. The former increases the total transport current and dominates in the graphene lightly doping condition. The latter decreases the total transport current and dominates in the graphene highly doping condition. Under infrared light, the tri-layer graphene photodetector also shows excellent repeatability and high responsivity (~2.5e^-4 A/W). It's believed that figuring out the photocurrent mechanism in graphene will help design extraordinary graphene photodetectors.