Silicon-based image sensors are attractive for applications in the near-infrared (NIR) range owing to their low-cost and high availability. However, novel approaches are required to enhance their light absorption, hindered by the silicon band gap. In this study, we proposed a light trapping strategy in a silicon absorption layer by plasmonic diffraction and reflection within a pixel to improve the sensitivity at a specific NIR wavelength for complementary metal-oxide semiconductor image sensors. The plasmonic grating diffracted light under the quasi-resonant condition of the surface plasmon polaritons. We simulated the silicon absorption efficiency for plasmonic diffraction combined with metal-filled trenches and a pre-metal dielectric (PMD) layer. Backward propagation light in silicon by a total internal reflection at the bottom decoupled with plasmonic grating. A single SiO2 protrusion was added at the silicon bottom to prevent decoupling by scattering the light in the silicon and trapping it within the pixel. In addition, the light transmitted to the PMD layer is reflected by the wiring layer used as a mirror. The photon confinement in silicon by these constructions improved the absorption by approximately 8.2 times at an NIR wavelength of 940 nm with 3-µm-thick. It is useful for NIR imaging system with active laser illumination.