The invention relates to a papermaking aid, and particularly discloses application of carboxymethyl chitin to serving as a retention and filtration aid. The carboxymethyl chitin is discovered to have favorable retention aiding and filtration aiding effects and water solubility, can adapt to acidic and neutral conditions, and is a renewable resource to be developed. The invention also discloses a retention and filtration aid system prepared from the carboxymethyl chitin and polymeric aluminum chloride; the retention and filtration aid system is discovered to have excellent retention aiding and filtration aiding performance. The invention also proposes that the retention aiding and filtration aiding effects are also different once the addition sequences of two reagents in the system are different. The application of the carboxymethyl chitin to serving as the retention and filtration aid has the advantages that (1) the carboxymethyl chitin is proposed to be applied to the retention and filtration aid for the first time, and the retention and filtration aid which is favorable in the retention aiding and filtration aiding effects, is renewable, is low in cost and is applicable to acidic and neutral papermaking is obtained; (2) the retention and filtration aid system which is prepared from the carboxymethyl chitin and the polymeric aluminum chloride is proposed, and the system has the excellent retention aiding and filtration aiding performance.