The invention discloses a lignin and a method for modifying a phenolic resin which is a ramification thereof. The method that the polymerization is performed for a plurality of times and the extent of polymerization is deepened gradually is adopted to improve the uniform stability, the inoxidability, the toughness and the intensity of the product, effectively recycle the wasted resources, lower the cost and make the industrialization easier. The invention is realized by the technical proposal which is as follows: the phenolic resin low polymer is prepared firstly; the lignin or a ramification thereof is added to polymerize in order to obtain a preformed polymer secondly; finally, phenol and formaldehyde are added to polymerize in order to obtain the high polymer. A molding material prepared by the phenolic resin can be widely applied to the manufacture of a low-voltage apparatus, an insulating structure and a product for daily life.