Drainage layer strategy is a common method for improving the filtration performance of a coalescing filter. In this study, we employed commercial, sub-high efficiency glass fibrous filters as drainage layers. The performance of high efficiency coalescing filters assembling the drainage layer was investigated experimentally. After drainage layer was assembled, the wet pressure drop at equilibrium stage was reduced and the pressure drop allocation exhibited obvious change. In addition, the influence of pore size, thickness and wettability of drainage layer on performance was evaluated. The pore size is a dominant factor in improving wet pressure drop at steady state. The wet pressure drop reduces gradually over pore size. The drainage layer with smallest pore size shows highest quality factor. Likewise, the thickness of the drainage layer also has a positive effect on wet pressure drop and quality factor. The coalescing filter assembling thickest drainage layer has lowest pressure drop and highest quality factor. The effect of superoleophilic drainage layer on wet pressure drop and quality factor of filter is negligible, and the superoleophobic drainage layer results in the enhancement of pressure drop and reduction of quality factor. For different coalescing filters with the same drainage layer, the smaller the pore size deviation between coalescing and drainage layer, the lower the wet pressure drop and the higher the quality factor.