The invention relates to a warm-mix asphalt additive and a preparation method thereof. The additive comprises the following components in percentage by weight: 87 to 98 percent of chemical modification latex, 0.5 to 12 percent of lubricative and peeling resistant cation surfactant and 0.1 to 2.0 percent of acid. The method comprises the following steps of: heating the chemical modification latex to the temperature of between 40 and 80 DEG C with stirring, and dripping 0.1 to 0.5 percent of acid slowly; adding the lubricative and peeling resistant cation surfactant, stirring fully, and dispersing uniformly; and continuing to drip 0.1 to 1.5 percent of acid slowly, stirring uniformly, keeping the temperature of between 40 and 80 DEG C and storing for later use. By the warm-mix asphalt additive and the preparation method, the viscosity of asphalt binders during mixing can be reduced effectively, the reduction performance of asphalt cements and adhesivity between the asphalt cements and aggregates can be improved, the water damage resistance and durability of warm-mix asphalt mixtures can be improved, the tensile strength of asphalt can be improved, and the rutting resistance, fatigue resistance and low temperature resistance of the warm-mix asphalt mixtures can be enhanced.