The development of filtration membranes with ultrahigh water flux for separation of oil-in-water emulsions by low energy consumption approach is currently an urgent demand. A novel nanocomposite membrane with superhydrophilic and underwater superoleophobic properties has been successfully prepared by one-step vacuum filtration of an aqueous graphene/SiO2 dispersion on a microfiltration substrate. Owing to the addition of SiO2 nanoparticles, the interlayer space of graphene oxide (GO) layers is expanded, enabling this composite membrane to separate a variety of oil-in-water emulsions with ultrahigh water flux (> 4550 Lm(-2) h(-1) bar(-1)), a 2-order-of-magnitude improvement as compared to pure GO membrane, and also superior compared to most commercialized membranes. The proposed convenient, time-saving (< 10 min) and low cost strategy endows possibility of massive production and shed light on commercial applications of water treatment of oil-in-water wastes in industry and daily life.