The present disclosure provides a tightly mountable optical fiber adapter which has a plurality of mating parts formed at front and rear sides of a base of a shell member thereof, and the base is combined with a base plate and two opposite side panels, each side panel is extended to form a first elastic part, and ends of second elastic parts are respectively outwardly bent and extended to form stop parts whose inner sides are bent towards the first elastic part and extended to form third elastic parts. While shell member is transversely mounted into an installing hole, first elastic parts are abutted and deformed to cross installing hole to clasp back side, second elastic parts are located in installing hole, so that shell member cooperating with mounting spring clip can be tightly fastened in installing hole, whereby the entire structure can be more securer.