The invention discloses preparation of biomass char doped polyacrylonitrile (PAN)-c nanofiber felt, which comprises the steps of crushing, treating and uniformly dispersing a plant root system material in a polyacrylonitrile spinning solution to form a PAN-c spinning solution, and coating the surface of a stainless steel net with the PAN-c spinning solution using an electrostatic spinning instrument to form the PAN-c nanofiber felt. Experiments show that the prepared nanofiber felt has special wettability of super-affinity and super-lipophilicity in air, super-lipophobicity in water and super-hydrophobicity in oil, is applicable to separation of various types of immiscible oil-water mixtures and has the advantages of stable performance, high separation efficiency and high penetration pressure, and the oleophobic separation efficiency in the water and hydrophobic separation efficiency in the oil are above 99.8%. In addition, raw materials of the prepared nanofiber felt are cheap and easy to obtain, the preparation is simple in preparation process, convenient to operate and low in cost, and large-scale production and application can be achieved.