Dialdehyde carboxymethyl cellulose (DACMC) was synthesized by oxidizing carboxymethyl cellulose (CMC) with periodate NaIO₄ at low pH value,and the effects of pH value on the oxidation degree of CMC,as well as on the crystal structure,morphology and thermal stability of DACMC,were investigated by means of FT- IR,¹³C NMR,XRD and AFM.The results show that (1) pH value significantly and irregularly affects the oxidation degree of DACMC,and the oxidation degree reaches its maximum at pH 1.5,with an aldehyde content of 54.35%; (2) carbonyl groups are introduced into the molecular chain of CMC,which is verified by FT- IR and ¹³C NMR results; (3) the crystallinity of DACMC is higher than that of CMC and increases with pH value; (4) due to the break of glucopyranose rings，the molecular chain of DACMC becomes softer,entangles with each other more compactly and arranges more regularly,while the Z dimension of DACMC surface decreases with the increase of aldehyde content; and (5) the thermal stability of DACMC is poorer than that of CMC,especially in the condition of pH 1.5.