Activated carbon (AC) particles, as some of the most widely used sorbents, have an indispensable position in environmental remediation. Although AC surface adsorption was extensively investigated, its internal nanostructure has not been well elucidated. Exploitation of the internal nanostructure for pollution abatement is worthwhile pursuing to expand AC application field. Herein, the internal nanostructures of AC particle are revealed by step-by-step exfoliation and characterization, indicating that an AC particle is composed of multilevel nanostructures including carbon nanospheres and graphene-like layers to a certain extent. AC particles can be transformed into an ultrafiltration membrane through a newly developped structure exfoliation and reconstruction processes under surface regulation without additional cross-linkers. The pure AC membrane (ACM) preserves the adsorption capacity of AC particles, and further endows AC with the function of membrane separation. The ACM shows excellent stability, strong adsorption capacity (252.4 mg/g for methylene blue), narrow sieving pores distribution (5–10 nm), low-fouling property for cycling use. The considerable water flux is 12–27 times higher than commercial ultrafiltration membrane. Dual-function performances of adsorption and separation make the ACM high potential in water purification. This approach for particle-to-membrane transformation provides a new perspective for AC particle future utilization.