In order to realize the application requirements of ultra-narrowband filters in the field of optical communication and optical sensing, this paper proposes a design concept of introducing a coupled guided mode resonance in an asymmetric grating waveguide structure to complete the efficient transmission filtering for specific wavelengths. The filtering structure consists of two added subwavelength gratings stacked on a silicon-based waveguide with the same period but different filling factors. Light waves are incident vertically from the top of the composite gratings, and the asymmetric resonant coupling of multiple waveguide modes can be excited by adjusting the thickness and filling factor of the bottom grating. Numerical simulations show that extremely strong electric field enhancement can be generated using this symmetry-broken guided-mode resonant coupling. Under the premise of satisfying the high sideband rejection ratio, the structure not only realizes the ultra-narrowband filtering effect of 0.005 nm, but also has a high transmission efficiency of 99%.