The present invention provides an attachment base for a lighting unit of a decorative lighting string comprising an upper base member inserted with an LED lamp and mounted onto an lower base member; the upper base member comprising locking hooks extended downward from two lateral sides of a bottom portion thereon; the lower base member comprising buckle holes adjacent to a bottom portion on two lateral sides corresponding to the locking hooks; characterized in that inner bottom surfaces of rear of the two buckle holes on the lower base member comprise blocking plate; a distance between the blocking plate and the buckle hole is slightly greater than a thickness of the locking hook. With such blocking plates, once the locking hooks are secured onto the buckle hole, the locking hooks abut the blocking plate to prevent the looking hook to disengage from the buckle hole under external forces.