Disclosed are an additive of a cement composition including a cross-linked polycarboxylic copolymer and a cement composition including the same. More particularly, an additive of a cement composition including a polycarboxylic copolymer and/or a salt thereof, wherein the polycarboxylic copolymer is a copolymer of a monomer mixture including an alkoxy polyalkylene glycol mono(meth)acrylic acid ester-based monomer, a (meth)acrylic acid-based monomer, and a cross-linking agent containing 5 to 20 alkylene oxide groups and having two or more acrylate groups or vinyl groups, and a cement composition including the same are disclosed. The additive of the cement composition including the polycarboxylic copolymer according to the present invention controls a cross-linked structure and thereby initial sensitivity and retentivity are improved, and, accordingly, a cement composition having superior long-term workability may be formed.