Disclosed is a method for preparing a hot-melt adhesive composition and a hot-melt adhesive composition prepared thereby. The method comprises: preparing and mixing raw materials, including a butyl rubber, an ethylene propylene diene (EPDM) rubber, a styrene block copolymer, an amorphous poly-alpha-olefin (APAO) and a tackifier resin; and performing vacuum degassing of the mixed raw materials, wherein the styrene block copolymer comprises a styrene-isoprene-styrene (SIS) rubber. The hot-melt adhesive composition has high resistance in temperature cycles between low and high temperatures so as to maintain its physical properties, and thus has an excellent property of sealing a headlamp for a long period of time until the end of the lifespan of the headlamp. Also, the composition generates no gas in the sealed state of the headlamp, and thus does not create bubbles or voids and does not pose a water tightness problem.