Size-sieving is an effective strategy for emulsion separation. However, the poly-dispersed porous nature of conventional size-sieving materials compromises their separation performances. Distinctive from poly-dispersed porous size-sieving materials, microporous metal-organic frameworks (MOFs) have homogeneous and uniform microporous channels that are promising to present high porosity utilization with exceptional demulsifying capability for both microemulsion and nanoemulsion. However, the internal diffusion kinetics of microporous MOFs dominate the transport behaviors of liquid inside their microporous channels, which poses a major kinetics obstacle for achieving high separation flux. Herein, we developed composite fibers by using hierarchically fibrous structured collagen fibers (CFs) as transport kinetics-promotable channels and microporous MOFs (ZIF-8 or HKUST-1) in situ-grown on CFs as a size-sieving layer. CFs feature a capillary effect that induces fast liquid transport along the fibers, which alleviates the kinetics shortcomings of microporous MOFs, thus boosting the separation flux. A synergistic effect was proven to exist between the size-sieving merits of MOFs and capillary effect of CFs. The as-prepared composite fibers realized the ultrafast separation of surfactant-stabilized micro- and nanoemulsions, showing two orders of magnitude flux enhancement on those of commercial double-sided polyvinylidene fluoride membrane and double-sided polytetrafluoroethylene membrane. These investigations demonstrated a new conceptual strategy for developing size-sieving materials with high flux.