Paper discoloration,which is related to the aging of cellulose,dust,mold,watermarks,painting pigments,etc.,affects the durability and appearance of paper relics and books in libraries.This paper discusses the causes of paper discoloration,based on the analysis of paper components, such as cellulose,hemicellulose,lignin and resin.To restore the original color of the paper,chromophoric groups must be destroyed either by opening conjugated double bonds,by separating chromophoric and auxochromic groups,by changing the chemical structures of chromophoric groups,or by oxidation of double bonds or side chains to decrease the amounts of chromophoric materials.Nowadays,the most widely used methods in the restoration workshops are oxidation bleaching,reducing bleaching and light aqueous bleaching.The decoloration mechanism of different methods and their effects on paper relics are reviewed.By comparison,it can be seen that all bleaching methods cause some damage to the paper.Their application on paper relics should be selected carefully according to different conditions.After bleaching,the paper should be washed thoroughly.