A dismantling device and a method for dismantling a backlight unit are provided, and the dismantling method includes: placing a liquid crystal module that includes a liquid crystal panel and a backlight unit placed oppositely and connected by a bonding portion onto a supporting table; bringing a line cutting portion that includes two ends and a cutting line therebetween into a gap between the liquid crystal panel and the backlight unit, with at least one of the two ends and the liquid crystal module configured to be movable with respect to each other; and applying a cutting force upon the bonding portion with the line cutting portion along a plane where the bonding portion is located, so as to separate the backlight unit from the liquid crystal panel. This method decreases the force upon the backlight unit to reduce chances of damaging the backlight unit during dismantling the same.