In this work, polyaniline-coated carbon nanofiber with acid treatment (named CNFA@PANI) composite was synthesized and used to eliminate diclofenac from the water environment. Thanks to the abundant amino functional groups and benzene-ring structures, the as-prepared CNFA@PANI composite presented the excellent adsorption capacity (qe) of 202.9 mg/g and high distribution constant (Kd) value of 1.02×104 for diclofenac. The physical and chemical properties of the manufactured composites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transformed infrared spectroscopy (FT-IR). The novel adsorbent possessed rapid and excellent adsorption performance for diclofenac. Moreover, in the wide pH range, the CNFA@PANI possessed well adsorption capacities. Meanwhile, after five adsorption-desorption cycles, the composite still showed excellent reusability performance. Furthermore, quantum chemical theory calculations indicated the possible adsorption sites of the composite for diclofenac. In total, this work provides an easy method to design novel composites for the efficient removal of diclofenac from the water environment.