Solvent-free adhesive is a kind of environmental friendly adhesive which is safe, less material consumptive and just needs simple curing conditions. In this paper, the two components solvent-free polyurethane adhesive for flexible packaging was discussed, including its characteristics, composition, curing and adhesion mechanism and three modification methods. The curing mechanism of this adhesive is that crosslinking happens between –NCO and –OH which are contained in two components respectively. However, its adhesion mechanism is different when the adhesive is bonding with polyester film, paper, fabric, metal and other materials. The modification methods of two components solvent-free polyurethane adhesive include modification of its soft segment, modification with epoxy resin and modification with organosilicon and so on. The performance of adhesive can be improved through modification.