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 Qis 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 Qvalue and electric field strength increase. When the thickness drops to about 12 nm, the minimum line width is 0.98 nm and the maximum Qvalue 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.