The invention discloses a method for preparing carboxymethyl cellulose by waste paper serving as a raw material. The method includes the steps: drying waste paper pulp, cutting the dried waste paper pulp into pieces, adding sodium hydroxide solution, reacting mixture at the temperature ranging from 40 DEG C to 100 DEG C, filtering and washing the mixture to reach neutrality, and drying the mixtureto obtain refined cellulose; immerging the refined cellulose into ethanol water containing sodium hydroxide, performing quaternization at the temperature ranging from 25 DEG C to 45 DEG C to obtain alkaline cellulose; continuing to add ethanol water containing chloroacetic acid, heating mixture to reach the temperature ranging from 60 DEG C to 70 DEG C, adding ethanol water containing sodium hydroxide to serve as alkaline catalyst solution, and performing etherification reaction at the temperature ranging from 60 DEG C to 80 DEG C; adjusting pH (potential of hydrogen) to reach neutrality, filtering mixture in an extracting manner, washing and precipitating the mixture by the aid of ethyl alcohol, and drying the mixture to obtain the waste paper carboxymethyl cellulose. According to method, high-additional-value utilization of waste paper resources is achieved, product cost is reduced, environments are protected, and the method is low in cost, simple in operation process, high in product substitution and green and environmentally friendly in process.