Abstract: The control of the molecular weight of polyacrylonitrile and its distribution is important for carbon fiber preparation. Polyacrylonitrile was synthesized by photo-induced solution polymerization using acrylonitrile as the monomer and dimethyl sulfoxide as a solvent. The molecular weight and molecular weight distribution of the polyacrylonitrile were respectively determined by a viscosity method with an Ubbelohde viscometer and gel permeation chromatography. The functional groups on and the chain structure of polyacrylonitrile were analyzed by infrared spectroscopy and nuclear magnetic resonance spectroscopy. Results showed that with an increase of polymerization temperature or monomer concentration, the conversion of the monomer and the molecular weight increased. Compared with the traditional initiator-induced radical polymerization of acrylonitrile, photo-induced polymerization significantly narrows the molecular weight distribution. Polyacrylonitrile with both a high molecular weight and a relatively narrow molecular weight distribution was manufactured by photo-induced solution polymerization.