Abstract: This invention relates to an amphoteric copolymer, and the use of the copolymer to improve the fluidity and fluidity retention of cementitious materials. The chemical structure of the copolymer is as follows: wherein R1 is H or CH3; R2 is a hydrogen atom, or an alkyl group, a cyclic aliphatic group or an aryl group, having 1 to 10 carbon atoms; D is H or COOR3, R3 is a hydrogen atom, or an alkyl group, a cyclic aliphatic group or an aryl group, having 1 to 10 carbon atoms, or a cationic salt group; Z is an O atom or an NH group; A is a —COO group, a —SO3 group or an acid form; a, b, or c is an integer from 1 to 5000; and p and q are integers from 1 to 10.