The invention relates to the technical field of mixture refining with hydrocarbon as a main and relates to an adsorption separation device and an adsorption separation device method for improving a base oil viscosity index. An adsorption separation device comprises a pump, an adsorption tower, a fractionation tank, a buffer tank, a condenser and a recovery tank, the adsorption tower, the fractionation tank, the buffer tank, the condenser and the recovery tank are respectively and fixedly installed on a support, a first pipeline is fixedly connected with an inlet end of the pump, an outlet of the condenser is fixedly connected with the first pipeline through a second pipeline, and an outlet of the pump is fixedly connected with a lower shell side inlet of the adsorption tower through a third pipeline. According to the adsorption separation device and the adsorption separation method, the use is convenient, purposes of improving base oil viscosity and separating through matched use of the adsorption tower, the fractionation tank, the recovery tank and the condenser are achieved, and the adsorption device has the advantages of being simple in process operation, soft in technological condition, small in investment, recycled in adsorbing materials and free of pollution on environment, and improving the base oil viscosity index and oxidation stability.