La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) perovskite hollow fibre membranes were prepared via extruding and sintering technology. The original LSCF hollow fibres exhibited low oxygen fluxes due to the existence of three fully densified layers unfavourable for oxygen permeation. The membrane was further improved by surface modification via an acid-etching technique. Sulphuric acid and hydrochloric acid of various concentrations were employed to optimise the surface modification. The fibres were characterised by SEM and XRD and tested for air separation. Experimental results revealed that HCl or H₂SO₄ etching can create a new membrane structure and improve the oxygen fluxes by a factor of up to 5.9 or 18.6, respectively. Comparatively, H₂SO₄ was a better acid for controlling the reaction to yield a porous surface with higher surface area. Theoretical results indicate that bulk diffusion still greatly influences the oxygen permeation process of the modified membranes. Strategies for further improvement should be focused on development of asymmetric hollow fibres with reduced thickness in the central densified layer without sacrificing the porous surface area.