In this study, the effect of surface wettability on the coalescence of oil droplets and pressure drop within a fibrous filter were investigated numerically. We simulated the permeation and coalescence of an oil-in-water (O/W) emulsion through a fibrous filter using our direct numerical simulation model based on the phase-field model, the immersed boundary method, and a wetting model that assigns an order parameter to the fiber surface. To appropriate the actual phenomena inside the filter, the microporous structure obtained from X-ray computed tomography (CT) images were employed in the simulation. With a low surface wettability, the arrangement of closely attached fibers at the permeating side of the filter domain promoted droplet coalescence. With a high surface wettability, the arrangement of small interfiber spacing placed at the permeate side significantly enhanced droplet coalescence, owing to the formation of a liquid-bridged structure between the fibers at the permeate side. The pressure drop increased with the high surface wettability. The liquid bridges that formed between the fibers at the permeate side blocked the flow path of the permeating fluid, causing a higher pressure drop. A sparse/dense structure, such as the closely attached fibers and small interfiber spacings at the permeate side, promoted droplet growth. Although the uniform fiber arrangement reduced the number of uncoalesced droplets passing through a filter, the coalescence performance is not very high, and the pressure drop significantly increases because the enlarged droplets block the flow path.