The invention provides a sealant additive online adding device. The sealant additive online adding device comprises a material conveying pipeline, a reaction cylinder, a mixing device, a superchargingdevice, a condensing unit, a buffer device, a filling machine, a metering device and an additive storage tank, wherein the reaction cylinder, the mixing device, the supercharging device, the condensing unit, the buffer device and the filling machine are sequentially arranged on the material conveying pipeline; the mixing device is further provided with an additive feed port; a discharge port of the metering device is connected with the additive feed port of the mixing device; a discharge port of the additive storage tank is connected with a feed port of the metering device; and the mixing device is a multi-component mixing machine. Compared with the prior art, the provided device is sequentially provided with the reaction cylinder, the mixing device, the supercharging device, the condensing unit, the buffer device and the filling machine on the material conveying pipeline, the high temperature filling is avoided, the guarantee period of a product is prolonged, the energy consumption is reduced; moreover, an additive is added online, so that the additive is stably mixed with a base stock, the quality fluctuation of the product is small, and the production efficiency is high.