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In this paper, we predict an unexpected enhanced optical absorption (OA) phenomenon in an optical thin (60 nm) freestanding metallic grating. After introducing periodical back grooves to the grating, the absorption could be enhanced up to 95% for light incident from the topside, which is inhibited lower than 10% for light incident from the bottom side. Physically it is ascribed to the strong modulation effect of the surface plasmons (SPs)/or charge distribution on the back surface of the grating by the grooves. As a result, the reflection at the SPs resonant position is greatly inhibited. It indicates a new mechanism to achieve high OA in a freestanding subwavelength structure by directly controlling the SPs. More counterintuitively, the highly enhanced absorption will increase as the filling factor of the grating decreases, rather than decrease as the filling factor decreases. So even for a very small filling factor (0.5), unexpected high OA up to 95% could be attained at SPs resonance. The underlying physical mechanism is analyzed with a dipole moment description.