We designed a low-frequency (<500 Hz) sound attenuation absorber based on micro-slits and space-coiling channels. The absorber provides near-perfect absorption at an extremely low-frequency of 137 Hz, simultaneously featuring a deep sub-wavelength thickness down to λ /85 (29.5 mm). It is proved that the excellent absorption is mainly caused by the friction losses of acoustic waves in micro-slits and the increase of the effective propagation length. By adjusting folding numbers and partition panels, the resonance frequency of the absorber is easily tuned and near-total absorption is obtained under fixed thickness. We further broadened relative absorption bandwidth to 54% by coupling two unit cells exhibiting different properties. The absorption mechanism is investigated utilizing acoustic impedance together with the reflection coefficient in the complex frequency plane. The absorption properties were demonstrated analytically, numerically and experimentally. The high-efficiency absorption, sub-wavelength thickness, and easy fabrication suggest that the absorber could have potential applications in acoustic engineering.