The perfect absorption is achieved by the structure of a continuous metal film with symmetrical grating structure on both sides. The maximum absorption coefficient can reach 99.47% for a optimal structural parameters with a silver film thickness of 20 nm, a lattice constant of 400 nm, and a medium refractive index of 1.46. The full width of half maximum of the absorption line is about 2.53 nm, and the quality factor Q is 296.06. When the absorption is perfect, the reflection and transmission of the incident light are effectively suppressed, and the phase gradient of the absorption coefficient reaches a maximum value. The perfect absorption is determined by the long-range surface plasma polariton (LRSPP) with a little transmission loss, long propagation distance and deep penetration depth. And the electric field is mainly distributed outside the silver film with a standing wave distribution. As the thickness of the silver film decreases, the line width of the absorption spectrum gradually decreases, while the Q value and electric field strength increase. When the thickness drops to about 12 nm, the minimum line width is 0.98 nm and the maximum Q value is 760.0. The sharp absorption curve and very high quality factor at the perfect absorption can be used in the design and application of the highly sensitive micro-nano sensor.