The invention discloses a cement additive and a preparation method therefor. The cement additive is characterized by comprising the following components in percentage by weight: 35% to 60% of hardening agent, 15% to 35% of calcium chloride, 5% to 15% of polydimethylsiloxane, 5% to 10% of silica, 5% to 8% of water reducing agent, 5% to 8% of cellulose ether and 5% to 8% of polyvinyl alcohol. The cement additive has good wear resistant properties, less general cement consumption, and low usage cost.