A kind of surface plasmonic grating optical absorber with gradually varying geometrical parameters is designed, and its absorption characteristics are analyzed in detail by the two-dimensional finite-difference time-domain (2D-FDTD) method. Results show that the reflectivity can be effectively reduced and the absorption capacity can be enhanced by adjusting the depths of the grooves, the widths of the grooves and the thicknesses of the walls. By increasing the widths of the grooves linearly and reducing the thicknesses of the walls linearly from centre to the both sides at the same time, better absorption effect can be achieved. The shape of the reflection spectrum is different for different spatial full width at half-maximum of the incident light. When the full width at half-maximum is narrow, the absorption ability is stronger for shorter wavelength light. When the full width at half-maximum is wide, the absorption ability is stronger for longer wavelength light. This light absorber is expected to be applied in all-optical chips.