Here, we present a graphene-based long-wavelength infrared photodetector, for enhancing the infrared absorption of which the design consists of magnetic- and electric-plasmon resonators of metasurface to excite the graphene surface-plasmonic polaritons (SPPs). Through tuning the graphene Fermi energy to achieve the distinct resonances in a matching frequency, peak graphene absorbance exceeding 67.2% is confirmed, even when a lossy dielectric is used, and the field angle of view is up to 90°. If the graphene is of a different carrier mobility, then the absorption frequency is lockable, and the device always can keep the system absorbance close to 100 percent. The significantly enhanced graphene absorbance, up to ~29-fold that of a suspended graphene (general 2.3%), is attributed to the surface-plasmonic coupling between the magnetic and the electric resonances, as well as Fabry-Pérot interference of the coherent SPPs. The plasmonic cavity-mode model and equivalent-circuit method developed in this study will also be useful in guiding other optoelectronic device design.