In this paper, a graphene photodetector based on plasmonic gold nanoparticles in the form of the pyramid with square sides has been introduced. Graphene is used as a material with desired electronic properties for detection in the visible wavelengths. One of the major limitations of monolayer graphene is its low light absorption (about 2/3%) at visible spectrum. In the present work, we have improved the light absorption of a monolayer graphene employing plasmonic properties of the pyramid-shaped gold nanoparticles on top of graphene which enhances light absorption. In the our given structure, we have change the geometric parameters of the structure via optimization of light absorption. Simulation results demonstrate that the amount of light absorption is enhanced more than 11 times in comparison to the structure without nanoparticles. This would facilitate the application of monolayer graphene in optoelectronic devices at visible wavelengths.