The effect of drainage layer on the pressure drop evolution, oil distribution and separation performance was investigated experimentally. The results showed that there existed obvious oil film on the rear face of wettable layer to cause a sharp jump in pressure drop. The interface between wettable coalescing media and non-wettable drainage layer could facilitate the drainage of coalesced oil and ameliorate the oil distribution density near the interface to improve the droplet capture probability. And because of the drainage function of the interface, the saturation of this kind of bed was lower. Oil distribution density in wettable drainage layer was relatively more uniform than that in the front coalescing layer, while in the bed only consisting of non-wettable media there existed a sparse zone of oil distribution in the lower half of the bed and this may be one reason for the low separation efficiency of non-wettable bed. A fiber bed with the arrangement of non-wettable drainage layer following behind wettable coalescing layer had the best overall separation performance.