The synthesis principle of cellulose water retention agent is polymerization initiated by free radical and the grafting mechanism usually includes chain initiation, chain growth, chain termination, and oxidation. In the presence of initiator, the free radical is generated on the cellulose macromolecule skeleton firstly, and then with the help of chain transfer, the graft copolymer is formed by reacting with monomer. Applying N,N’-methylene bisacrylamide as crosslinker, potassium peroxydisulfate as free radical evocating agent, acrylic acid as grafting monomer, cellulose in corn straw biogas residue was purified by nitric acid-ethanol method to synthesize water retention agent. Through single factor experiment and orthogonal experiment, the synthesis process was optimized, and the absorbency, infrared spectra and surface morphology of obtained water retention agent in the experiment were characterized. The experimental result showed that the optimal process of corn straw residue water retention agent was as follows: the mass ratio of acrylic acid to actived straw residue at 7, neutralization degree at 70.0%, the dosage of initiator (mass ratio to the monomer) and crosslinker (mass ratio to the monomer) at 2.0%, 0.1%, respectively. Furthermore, 9 min microwave moderate heating then 1 min higher heating were needed in the continuous experiment. Under this experimental conditions, the straw residue water retention agent had stronger water absorbency, and the maximum water absorption rate was 382.22g/g as well as with a certain thermal stability. Infrared absorption spectrum showed that there were C—O stretching vibration peaks of acrylic at 1200cm-1 and O—H bending vibration zone of acrylic acid near 1350cm-1. Furthermore, the neutralized —COO- reduced the C=O in acrylic acid, and the absorption peak of C=O appeared at 1557.98cm-1. All of above indicate that grafting polymerization of corn straw residue cellulose water retention agent is successful.