Acoustic liners are the most effective passive solution to noise emission from ducts but still suffer from the drawbacks of narrow-band performances and thick structures for the conventional designs. Here, we present an acoustic metaliner capable of high-efficiency broadband sound attenuation under grazing flow. The metaliner is composed of a perforated plate and a metasurface consisting of a series of neck-embedded Helmholtz resonators (NEHRs). By tuning the nonlocal coupling among the NEHRs and the perforated plate, efficient broadband impedance modulation can be realized, leading to a strong dissipation of broadband sound energy under various speeds of grazing flow. We theoretically and experimentally investigate a well designed metaliner, which can strongly attenuate sound energy for a wide frequency range from 800 Hz to 3000 Hz, both in the absence of air flow and in the presence of grazing flows with speeds from 10 m/s to 60 m/s. Besides, the thickness of the metaliner is only 40.5 mm. Our work provides a route to reduce flow-related noise in a broad frequency range and may find applications in airplane propulsion systems, air-conditioning systems, and other mechanical systems with flow.