An earplug structure being a columnar body has a sound insulation portion at an end and a holding portion at the other end. The sound insulation portion includes first to fourth flaps which are bowl-shaped structures, and the first flap has a surface curvature from −6.275 to 5.167, and the second flap has a surface curvature from −6.211 to −4.233, and the third flap has a surface curvature from −4.53 to −4.335, and the fourth flap has a surface curvature from −3.963 to −3.872, and the first, second, third and fourth flaps come with different sizes that increase gradually from the first flap to the fourth flap. The flaps are designed with a bowl-shaped curvature which increases the contact surface between the earplug structure and a users auditory canal to achieve the effects of improving the close contact effectively and insulating the sound completely.
